Connect: SDK/Built-in Proxy Support for Mesh-Gateways

[banks]

Scope:

• Probably no RFC unless it ends up really complicated for some unforeseen reason.
• ~1week work

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The Connect native SDK and by extension the Built-In proxy have not been updated to use the new Discovery Chains features added in the imminent 1.6.0.

Both are for now intentionally going to remain limited to L4 concerns as we have no desire to build a complete L7 load balancing and routing solution into the SDK for now, but there are several parts of the 1.6.0 Discovery Chain work which need support there:

1. It should still work when Mesh-gateways are in use
2. We should support the (still in flight) ExternalSNI and ExternalSAN (issue coming soon) overrides but more generally use the server's decision about those things rather than assuming we know how to synthesise the expected values.

For now we should probably not include attempting to use service-resolver for failover since that is part of a broader issue about making Consul service discovery aware of the discovery chain and deprecating Prepared Queries.