api: enable selecting subset of services using rendezvous hashing

[shoenig]

This PR adds the ?choose query parameter to the /v1/service/<service> endpoint.

The value of choose is in the form <number>|<key>, number is the number
of desired services and key is a value unique but consistent to the requester
(e.g. allocID).

Folks aren't really expected to use this API directly, but rather through consul-template / template block
which will soon be getting a new helper function making use of this query parameter.

Example,

curl 'localhost:4646/v1/service/redis?choose=2|abc123'

Part of #12575

Because this also includes updating CT, the template part works now as well.

[mikenomitch]

Ignorable nit: Should we split the number and the key into different params? And then require that both are set?

[shoenig]

Will add e2e test + complete example in the docs in a followup PR

[jrasell]

LGTM, this will be really useful! Minor inline question, but as mentioned, I feel I know the answer already.

[jrasell]

I think I already know the answer to this, but would it be worth logging the error content for cluster operators so this is available somewhere to lookup alongside the generic error return message?

[shoenig]

Well the only intended user is template/consul-template which will always be correct, though I could see the benefit for someone writing their own API client. If we do start logging RPC failures though we should probably do it at all the err points for consistency

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