reverse connection: reverse connection bootstrap extension

[basundhara-c]

Commit Message: The "envoy.bootstrap.reverse_connection" bootstrap extension is the central part of the reverse connection workflow and brings up several reverse connection entities described in this github issue. This PR is strictly tied with #37368.

Additional Description: The following reverse connection entities are relevant on the initiator envoy side:
A ReverseConnRegistry singleton is defined as part of the bootstrap extension which holds a TLS slot. The TLS slot holds a functor that creates a RCThreadLocalRegistry, which wraps over all the thread local entities that need to be initialized for reverse connections, in particular:

• The ReverseConnectionInitiator is a thread local entity created unique for each Listener Tag. On being created, the RCInitiator initiates reverse connections to clusters based on metadata present in the listener config. Upon connection closure, the owning RCInitiator is invoked to re-initiate connections.
• The ReverseConnectionManager is a a single thread local resource that manages the lifecycle of all ReverseConnectionInitiators. This is initialized as part of the RCThreadLocalRegistry and stores information about ReverseConnectionInitiator and the connections owned by them, and is called upon to invoke RCInitiators and replenish connections in case of closure.

The following entities are relevent on the responder envoy side:

• The ReverseConnectionHandler is a thread local socket manager that, on the responder envoy side, stores mappings of initiator node/cluster information with the corresponding reverse connection sockets.
• When a request arrives that needs to be sent over a reverse connection, the allocateConnPool API invokes the RCHandler to pick a cached socket for the host to sent the request.
• The Conn Pool then creates a ReversedClientConnectionImpl over the socket. The ReversedClientConnectionImpl skips connecting to the upstream host (because a socket is available) and calls the RCHandler to cleanup in case of socket closure. Note that re-initiaiton in case of closure is done only on the initiator envoy.

Risk Level: Low
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[repokitteh-read-only]

Hi @basundhara-c, welcome and thank you for your contribution.

We will try to review your Pull Request as quickly as possible.

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Caused by: #37819 was opened by basundhara-c.

see: more, trace.