PMM-4326 Use a shared GRPC connection

[ademidoff]

PMM-4326

Link to the Feature Build: SUBMODULES-4123

Refactored the Register*HandlerFromEndpoint to use a shared connection.

Changes made:

• Created a shared gRPC connection before registering handlers
• The services register now via Register*Handler instead of Register*HandlerFromEndpoint
• Added proper cleanup - the connection closes when the context is done (matching the pattern from Register*HandlerFromEndpoint)

All services now use a shared connection. The connection is created once and reused, avoiding the extra dials for every other service.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 25 lines in your changes missing coverage. Please review.
✅ Project coverage is 45.74%. Comparing base (c92fe2d) to head (ad34459).
⚠️ Report is 2 commits behind head on v3.

Files with missing lines	Patch %	Lines
managed/cmd/pmm-managed/main.go	0.00%	25 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##               v3    #4770      +/-   ##
==========================================
+ Coverage   45.68%   45.74%   +0.06%     
==========================================
  Files         364      364              
  Lines       37759    37747      -12     
==========================================
+ Hits        17249    17267      +18     
+ Misses      18851    18825      -26     
+ Partials     1659     1655       -4     

Flag	Coverage Δ
managed	46.21% <0.00%> (-0.01%)	⬇️

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