Backport of [OSS] gRPC Blocking Queries into release/1.15.x

[hc-github-team-consul-core]

Backport

This PR is auto-generated from #17426 to be assessed for backporting due to the inclusion of the label backport/1.15.

🚨

Warning automatic cherry-pick of commits failed. If the first commit failed,
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The person who merged in the original PR is:
@DanStough
This person should manually cherry-pick the original PR into a new backport PR,
and close this one when the manual backport PR is merged in.

merge conflict error: POST https://api.github.com/repos/hashicorp/consul/merges: 409 Merge conflict []

The below text is copied from the body of the original PR.

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Description

This PR adds blocking support to gRPC endpoints used for peering. Currently, these poll at a constant 1s interval. This is too slow for mesh gateway updates when using PeerThroughMeshGateways and too fast for steady-state updates that may trigger disco-chain compilation.

Testing & Reproduction steps

• CI automated test
• You can manually curl the HTTP endpoints using blocking query semantics and observe updates only on change.

PR Checklist

• updated test coverage
• external facing docs updated
• appropriate backport labels added
• not a security concern

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Overview of commits

• 11b1dd2 - 5ceef20

[hashicorp-cla]

All committers have signed the CLA.

[github-team-consul-core-pr-approver]

Auto approved Consul Bot automated PR

[DanStough]

This is a straight cherry-pick of commits already reviewed in ENT.