feat: make it possible to deifine a feature in Vertex AI FeatureStore

[modular-magician]

Part of hashicorp/terraform-provider-google#9298
Part of hashicorp/terraform-provider-google#12307

Add vertex_ai_featurestore_entitytype_feature to enable the google-beta provider to define a feature in Vertex AI FeatureStore. GoogleCloudPlatform/magic-modules#6565 is a PR to promote the other featurestore resources to GA from beta. If we merge PR#6565 first, it'd be nice to update this PR to use GA instead of beta too.

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vertex_ai_featurestore_entitytype_feature

Derived from GoogleCloudPlatform/magic-modules#6568