[gRPC] Add GetBackup() endpoint

[rzvoncek]

Fixes #661 .

[sonarcloud]

SonarCloud Quality Gate failed.   

0 Bugs
0 Vulnerabilities
1 Security Hotspot
5 Code Smells

No Coverage information
0.0% Duplication

Catch issues before they fail your Quality Gate with our IDE extension SonarLint

[codecov]

Codecov Report

Merging #684 (1ab28a0) into master (dca04bc) will increase coverage by 0.02%.
The diff coverage is 54.87%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #684      +/-   ##
==========================================
+ Coverage   77.85%   77.87%   +0.02%     
==========================================
  Files          52       52              
  Lines        4516     4552      +36     
  Branches      919      919              
==========================================
+ Hits         3516     3545      +29     
- Misses        968      976       +8     
+ Partials       32       31       -1     

Files	Coverage Δ
medusa/service/grpc/client.py	72.34% <66.66%> (-0.61%)	⬇️
medusa/service/grpc/medusa_pb2_grpc.py	64.17% <57.14%> (-0.83%)	⬇️
medusa/service/grpc/server.py	81.08% <82.92%> (+2.06%)	⬆️
medusa/service/grpc/medusa_pb2.py	22.00% <4.00%> (-1.92%)	⬇️

... and 1 file with indirect coverage changes