xds: add singleton subscription xDS-TP-based config-sources support

[adisuissa]

Commit Message: xds: add singleton subscription xDS-TP-based config-sources support Additional Description: Followup to #39781, as part of #24773. This PR adds the framework for singleton subscriptions to resources that are defined using the xDS-TP config-sources in the bootstrap. Next is to convert the current xDS singletons (EDS, RDS, etc.) to use this functionality.

Risk Level: low - not used Testing: Added unit tests. Docs Changes: N/A Release Notes: N/A Platform Specific Features: N/A

Part of #24773.

[repokitteh-read-only[bot]]

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:cat:

Caused by: https://github.com/envoyproxy/envoy/pull/39943 was opened by adisuissa.

see: more, trace.

[adisuissa]

/assign @nezdolik @tonya11en @yanavlasov

[adisuissa]

Thanks for the review @nezdolik !

[yanavlasov]

LGTM. Waiting for LGTM from @nezdolik as well.

/wait-any

[adisuissa]

/retest