Replace existing REST APIs with gRPC

[shubham4443]

Description

This PR is the first step of replacing all the existing APIs which are written in REST architectural style with gRPC. gRPC has benefits over REST such as speed, communication protocol (gRPC uses HTTP/2).

Please note that -

1. This is still at a proof-of-concept stage.
2. We are beginning with executor first.
3. generate-proto.sh has protobuf related commands for reference.
4. executor.proto has a sample UnaryEcho service for testing purpose.
5. the executor gRPC server is running at port 50051 along with the HTTP server. This port is exposed on the pod as well on the service.

[codecov]

Codecov Report

Merging #2706 (783617d) into main (a9d5542) will decrease coverage by 1.17%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main    #2706      +/-   ##
==========================================
- Coverage   21.12%   19.96%   -1.17%     
==========================================
  Files          65       64       -1     
  Lines        7215     7094     -121     
==========================================
- Hits         1524     1416     -108     
+ Misses       5577     5572       -5     
+ Partials      114      106       -8     

Flag	Coverage Δ
unittests	19.96% <0.00%> (-1.17%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pkg/router/functionHandler.go	10.94% <0.00%> (-0.13%)	⬇️
pkg/fission-cli/util/util.go	2.47% <0.00%> (-0.07%)	⬇️
pkg/poolcache/poolcache.go
pkg/utils/serviceaccount.go	78.60% <0.00%> (+0.70%)	⬆️

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