Add tags field to Alloydb Cluster for TagsR2401

[SamanthaMathews]

Add tags field to cluster resource to allow setting tags on cluster resources at creation time.
b/365030846

Release Note Template for Downstream PRs (will be copied)

alloydb: added `tags` field to `alloydb_cluster` to allow setting tags for clusters at creation time

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[roaks3]

@SamanthaMathews it looks like this is still in draft mode, so just let me know when you are ready for a review

[c2thorn]

Discussing with @SamanthaMathews offline about this PR and the go rewrite migration