Title: Generic xDS resource manager

Description:
Currently components that use xDS resources directly interact with the subscription messages, and in part need to adhere to the xDS-protocol and the transport-layer. This could lead to inconsistencies in the ways that different xDS services behave (for example, #13009, #16350, #12061).

We propose xDS-Manager, that will be the entry point for xDS-resource subscriptions that will:

• Decouple the xDS-transport layer from the xDS-data layer, and ensure the correct usage of the xDS-protocol.
• Serve as a single component that manages the connections with external config sources.
• Allow internal components (including extensions) to fetch resources using xDS - including arbitrary resource types over ADS.
• Ease the introduction of new xDS types and ensure they adhere to the xDS-protocol.
• Manage the fetched resources lifetime.

The design doc can be found here.

This proposal may address or improve the following issues (among others):

• xds: Consistency while handling CDS updates and EDS #13009 - ensuring that CDS updates are using the latest EDS resources.
• Proposal: add config blob discovery #11547 - blob discovery.
• xDS resource patching #8400 - resource patching (partial support).