DEV: Prepare new structure for migrations-tooling (#26631)

* Moves existing files around. All essential scripts are in `migrations/bin`, and non-essential scripts like benchmarks are in `migrations/scripts`
* Dependabot configuration for migrations-tooling (disabled for now)
* Updates test configuration for migrations-tooling
* Shorter configuration for intermediate DB for now. We will add the rest table by table.
* Adds a couple of benchmark scripts
* RSpec setup especially for migrations-tooling and the first tests
* Adds sorting/formatting to the `generate_schema` script

migrations/config/gemfiles/README.md

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+## Gemfiles for migrations-tooling
+
+This directory contains Gemfiles for the migration related tools.
+
+Those tools use `bundler/inline`, so this isn't strictly needed. However, we use GitHub's Dependabot to keep the
+dependencies up-to-date, and it requires a Gemfile to work. Also, it's easier to test the tools with a Gemfile.
+
+Please add an entry in the `.github/workflows/dependabot.yml` file when you add a new Gemfile to enable Dependabot for
+the Gemfile.
+
+#### Example
+
+```yaml
+  - package-ecosystem: "bundler"
+    directory: "migrations/config/gemfiles/convert"
+    schedule:
+      interval: "weekly"
+      day: "wednesday"
+      time: "10:00"
+      timezone: "Europe/Vienna"
+    versioning-strategy: "increase"
+```

migrations/config/gemfiles/common/Gemfile

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+# frozen_string_literal: true
+
+source "https://rubygems.org"
+
+# the minimal Ruby version required by migration-tooling
+ruby ">= 3.2.2"
+
+# `activesupport` gem needs to be in sync with the Rails version of Discourse, see `/Gemfile`
+gem "activesupport", "< 7.1", require: "active_support"
+
+# for SQLite
+gem "extralite-bundle",
+    "~> 2.8",
+    require: "extralite",
+    github: "digital-fabric/extralite"
+gem "lru_redux", "~> 1.1", require: false
+
+# for communication between process forks
+gem "msgpack", "~> 1.7"
+
+# for CLI
+gem "colored2", "~> 4.0"
+gem "thor", "~> 1.3"
+
+# auto-loading
+gem "zeitwerk", "~> 2.6"

migrations/config/intermediate_db.yml

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+## Configuration options for the base intermediate schema generator
+##
+## After modifying this file, regenerate the base intermediate schema
+## by running the `generate_schema` script.
+
+# Default relative path for generated base schema file.
+# An absolute path can also be provided to the script as the first CLI argument.
+# If the CLI argument is present, it takes precedence over the value specified here.
+output_file_path: "../db/schema/100-base-schema.sql"
+
+## Tables to include in the generated base intermediate schema.
+##
+## Available table options:
+## virtual:     Boolean. Enables the inclusion of a table in the schema solely based.
+##              on the provided configuration. A virtual table does not need to be available in the core schema.
+## ignore:      List of columns to ignore. Convenient if most of the table's column are needed.
+##              Usage is mutually exclusive with the `include` option. Only one should be used at a time.
+## include:     List of columns to include. Convenient if only a few columns are needed.
+##              Usage is mutually exclusive with the `include`` option. Only one should be used at a time.
+## primary_key: Literal or list of columns to use as primary key.
+## extend:      List of objects describing columns to be added/extended.
+##              The following options are available for an "extend" object:
+##                name:    Required. The name of the column being extended.
+##                is_null: Specifies if the column can be null.
+##                type:    Column type. Defaults to TEXT.
+## indexes:     List of indexes to create. The following options are available for an "index" object:
+##                name: Index name.
+##                columns: List of column(s) to index.
+tables:
+  users:
+    ignore:
+      - flag_level
+      - last_emailed_at
+      - last_posted_at
+      - last_seen_reviewable_id
+      - password_algorithm
+      - password_hash
+      - salt
+      - secure_identifier
+      - seen_notification_id
+      - username_lower
+
+## Schema-wide column configuration options. These options apply to all tables.
+## See table specific column configuration options above.
+##
+## Available Options:
+## ignore:  List of core/plugin table columns to ignore and exclude from intermediate schema.
+columns:
+  ignore:
+    - updated_at