[Tag System]
This CL includes 2 parts:
Add classes related to "tag"
Resource: is the collective name of the nodes that provide various service capabilities in Doris cluster.
Tag: A Tag consists of type and name.
TagSet: TagSet represents a set of tags.
TagManager: maintains 2 indexes:
one is from tag to resource.
one is from resource to tags
ISSUE #1723
Using JSON as serialization methods of metadata
Introduce GSON library to serialize the new classes mentioned above.
ISSUE #2415#2389
GSON's version is updated to 2.8.6