Add Additional gRPC service handler hook

[katrogan]

TL;DR

Extends the plugin registry with a configurable gPRC service handler hook to serve additional endpoints on the flyteadmin gRPC server.

Type

• Bug Fix
• Feature
• Plugin

Are all requirements met?

• Code completed
• Smoke tested
• Unit tests added
• Code documentation added
• Any pending items have an associated Issue

Complete description

^^^

Tracking Issue

N/A

Follow-up issue

NA

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (fbdb162) 59.05% compared to head (c832e7b) 60.65%.

❗ Current head c832e7b differs from pull request most recent head e9aa342. Consider uploading reports for the commit e9aa342 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #620      +/-   ##
==========================================
+ Coverage   59.05%   60.65%   +1.59%     
==========================================
  Files         171      171              
  Lines       16499    13469    -3030     
==========================================
- Hits         9743     8169    -1574     
+ Misses       5905     4449    -1456     
  Partials      851      851              

Flag	Coverage Δ
unittests	?

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Files	Coverage Δ
plugins/registry.go	100.00% <ø> (ø)

... and 157 files with indirect coverage changes

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[pmahindrakar-oss]

@katrogan can you may be add an example for the community on why would somebody try to do this extension . May be through some usecases which can be done in OSS

[katrogan]

@pmahindrakar-oss sure I can save that for a follow-up

btw the monorepo is being finalized today - so I will need to add a duplicate PR there once we've cut over exclusively, but thanks for the review

[katrogan]

closing in favor of flyteorg/flyte#4171