[GR-53334] Refactor tags into tag groups.

[graalvmbot]

Currently CI jobs have an optional tags field.
These tags are used only by the implementation of optional post merges.

To be able to use tags for different services, with different semantics, it would be nice if tags would be changed from being a list of strings to being a list of objects.

Example:

current:

"tags": ["bench-compiler"]

This PR refactors it in the following format:

"tags": {
  "opt_post_merge": ["bench-compiler"]
}