Convert inboundServices to an enum.

[modular-magician]

This will make the documentation better and more readable.

Release Note Template for Downstream PRs (will be copied)

appengine: converted `google_app_engine_standard_app_version`'s `inbound_services` to an enum array, which enhances docs and provides some client-side validation.

Fixes #6904.

Derived from GoogleCloudPlatform/magic-modules#3820

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