Filter Chain Metadata

*Description*:

Envoy Gateway supports injection of Gateway API resource metadata into xds metadata for observability and configuration analysis purposes.

Currently, the following are supported:
- Route metadata (from xRoute)
- VHost metadata (from Gateway)
- LocalityLBEndpoints metadata (from Service, Backend, ... )

Envoy supports filter-chain level metadata as well. Recently, the metadata access log operator was extended to support filter chain metadata: https://github.com/envoyproxy/envoy/pull/42685

This metadata can be useful for access log enrichment in several cases:
- Non-HTTP xRoute (TCP, TLS, ... ) map to filter chains
- HTTPS route listener logs: e.g. when a TLS connection fails (TLS parameter mismatch, client certificate validation failure, ... ), an HTTP access log is not emitted. However, a listener access log can still provide metadata based on filter chain metadata (assuming SNI matching succeeded).

Proposal:
- Create filter chain metadata based on
* Gateway and Listener
* Non-HTTP xRoute

Notes:
- Typically, the amount of filter chains is smaller than the amount of routes, backends, etc. So, XDS should not be inflated significantly.
- Filter chains can be added/removed/update independently of the listener, so adding metadata should not create undesired listener draining.

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