connect: expose an API endpoint to compile the discovery chain

[rboyer]

Fixes #6213
Fixes #6255

• change how DiscoveryTarget serializes to be SNI-based instead (comment)
• pull all target information into a toplevel map keyed by the ID value and reference them from nodes by that ID (like how node transitions work). This removes the need for DiscoveryTarget to be TextMarshalled
• /v1/discovery-chain/<service>: GET lets you pass the evaluationDC as an URL parameter but nothing else more complex. Anything else goes over POST (like overrides)
• don't expose evaluationNamespace over the API for now
• add endpoint tests
• docs (moved to add docs for /v1/discovery-chain endpoint #6273)
• api parallel copies of structures
• request chain via cache

[banks]

Looks great. I have a couple of thoughts for minor changes.

It also needs docs - I'd like to see what the output looks like in practice as it's a little hard to imagine just from the structs being used but I guess we'd see that in docs.

Also API package equivalents are also important not to forget before we land and close the issue.

[mkeeler]

Looks good to me. Just a few really minor requests.

[mkeeler]

LGTM

Matt approved instead as Paul is OOO.