feat: GrpcSource for ETL ng

[godexsoft]

For #1596 and #1597

This is a slightly modified version of the existing GrpcSource and a renamed AsyncCallData.
Instead of writing to the database directly from the async downloader we introduce an interface for observing the initial ledger load which allows us to separate the downloading from the writing.

The included unit tests are inspired by the existing etl/GrpcSourceTests and use the same approach with the mocked gRPC server.

[codecov]

Codecov Report

Attention: Patch coverage is 73.05389% with 45 lines in your changes missing coverage. Please review.

Project coverage is 71.71%. Comparing base (9dc322f) to head (0594ecb).
Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/etlng/impl/AsyncGrpcCall.cpp	72.00%	10 Missing and 11 partials ⚠️
src/etlng/impl/GrpcSource.cpp	71.64%	8 Missing and 11 partials ⚠️
src/web/Resolver.cpp	82.60%	2 Missing and 2 partials ⚠️
src/web/dosguard/WhitelistHandler.hpp	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1745      +/-   ##
===========================================
- Coverage    71.71%   71.71%   -0.01%     
===========================================
  Files          292      295       +3     
  Lines        12487    12649     +162     
  Branches      6359     6471     +112     
===========================================
+ Hits          8955     9071     +116     
- Misses        1848     1870      +22     
- Partials      1684     1708      +24     

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[kuznetsss]

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