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/db/README.md

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+# Intermediate Database
+
+TODO: Document stuff here.
+
+## Database Schema

migrations/db/schema/001-schema_migrations.sql

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+CREATE TABLE schema_migrations
+(
+    path       TEXT     NOT NULL PRIMARY KEY,
+    created_at DATETIME NOT NULL,
+    sql_hash   TEXT     NOT NULL
+);

migrations/db/schema/002-config.sql

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+CREATE TABLE config
+(
+    name  TEXT NOT NULL PRIMARY KEY,
+    value TEXT NOT NULL
+);

migrations/db/schema/003-log_entries.sql

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+CREATE TABLE log_entries
+(
+    created_at DATETIME NOT NULL,
+    type       TEXT     NOT NULL,
+    message    TEXT     NOT NULL,
+    exception  TEXT,
+    details    TEXT
+);

migrations/db/schema/100-base-schema.sql

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+/*
+ This file is auto-generated from the Discourse core database schema. Instead of editing it directly,
+ please update the `schema.yml` configuration file and re-run the `generate_schema` script to update it.
+*/
+
+CREATE TABLE users
+(
+    id                        INTEGER  NOT NULL PRIMARY KEY,
+    active                    BOOLEAN  NOT NULL,
+    admin                     BOOLEAN  NOT NULL,
+    approved                  BOOLEAN  NOT NULL,
+    created_at                DATETIME NOT NULL,
+    staged                    BOOLEAN  NOT NULL,
+    trust_level               INTEGER  NOT NULL,
+    username                  TEXT     NOT NULL,
+    views                     INTEGER  NOT NULL,
+    approved_at               DATETIME,
+    approved_by_id            INTEGER,
+    date_of_birth             DATE,
+    first_seen_at             DATETIME,
+    flair_group_id            INTEGER,
+    group_locked_trust_level  INTEGER,
+    ip_address                TEXT,
+    last_seen_at              DATETIME,
+    locale                    TEXT,
+    manual_locked_trust_level INTEGER,
+    moderator                 BOOLEAN,
+    name                      TEXT,
+    previous_visit_at         DATETIME,
+    primary_group_id          INTEGER,
+    registration_ip_address   TEXT,
+    silenced_till             DATETIME,
+    suspended_at              DATETIME,
+    suspended_till            DATETIME,
+    title                     TEXT,
+    uploaded_avatar_id        INTEGER
+);