[Internal] Add support for asynchronous data plane token refreshes

[renaudhartert-db]

DO NOT MERGE: api.go and impl.go are here to help with review and will be removed before merge. These are supposed to be added later on via code generation.

What changes are proposed in this pull request?

This PR is a step towards enabling asynchronous refreshes of data plane tokens.

The PR introduces a new interface dataplane.EndpointTokenSource which returns data plane tokens for a given endpoint and authorization detail. The default implementation relies on auth.NewCachedTokenSource to wrap the underlying TokenSources.

Note: async refreshes are disabled at the moment and will be enabled in a follow-up PR.

About Config.GetTokenSource():

My first attempt to implement GetTokenSource() returned an error if the TokenSource could not be built (e.g. because of config initialization errors). This didn't work because:

1. service constructors do not return errors and thus had to panic;
2. service constructors are called systematically as soon as the WorkspaceClient is created — sometimes with a config that is not compatible with GetTokenSource().

Returning a TokenSource provider instead of a TokenSource would have solved the problem but did not feel right. The current solution (i.e. return an ever-failing token source) is the cleanest solution I could think off.

How is this tested?

Complete unit test coverage of the new package.

[github-actions]

If integration tests don't run automatically, an authorized user can run them manually by following the instructions below:

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

Inputs:

• PR number: 1142
• Commit SHA: 2a236927e1b4c64e30125245c00d858f40a407f4

Checks will be approved automatically on success.