I opened this because I wanted to discuss / get some feedback on a few things. I am thinking I can split the rest of this work into 2 PRs (Maybe 3?). Contents would be as follows:

PR 1:

• Transport socket class
• Transport socket unit tests / test utils
• DownstreamAddresses transport socket option struct
• Refactor of listener filter to use common constants

PR 2:

• Transport socket config / config factory
• Transport socket API
• Transport socket Integration tests (Not in this PR yet)
• TCP Proxy API update for option to set down addrs socket option (Not in this PR yet)
• TCP Proxy update to set downstream addr option (Just hacked in for now so I could test things out)
• Any needed TCP Proxy integration / unit tests (Not in this PR yet)
• Documentation (Not in this PR yet)

PR 3:

• Performance pass?

This branch currently has PR 1 and some parts of what would be PR 2 in it. I'd just move the PR 1 pieces to a separate branch. Thoughts? I've also left a comment with a question on some code.

@alyssawilk @mattklein123

edit - Just noticed that v2 xDS is now frozen. So I'll have to update that.