[Internal] Decouple serving and oauth2 package

[parthban-db]

What changes are proposed in this pull request?

Package oauth2 contains a DataPlaneService that is not intended for users to use directly. This service is not auto-generated and is only used in the serving package. To remove the dependency between the serving package and the oauth2 package, this PR moves the DataPlaneService from the oauth2 to the serving package.

Note: serving package still depends on the oauth2 package for DataPlaneInfo struct. This dependency cannot be removed right now as it is in the OpenAPI.

How is this tested?

Existing unit tests

[github-actions]

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Trigger:
go/deco-tests-run/sdk-go

Inputs:

• PR number: 1110
• Commit SHA: 4399da5c6a0baed196b4c6ff8f285a1883d7aa96

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