Built-in support for free-format client-generated IDs

[bart-degreed]

Renames MongoIdentifiable to HexStringMongoIdentifiable and adds FreeStringMongoIdentifiable.

QUALITY CHECKLIST

• Changes implemented in code
• Complies with our contributing guidelines
• Adapted tests
• Documentation updated

[codecov]

Codecov Report

Merging #14 (5128bb5) into master (674889e) will increase coverage by 0.46%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #14      +/-   ##
==========================================
+ Coverage   60.09%   60.55%   +0.46%     
==========================================
  Files          16       17       +1     
  Lines         426      431       +5     
==========================================
+ Hits          256      261       +5     
  Misses        170      170              

Impacted Files	Coverage Δ
...re.MongoDb/Resources/HexStringMongoIdentifiable.cs	100.00% <ø> (ø)
...s/Internal/HideRelationshipsSparseFieldSetCache.cs	100.00% <100.00%> (ø)
...DotNetCore.MongoDb/Repositories/MongoRepository.cs	96.89% <100.00%> (ø)
...e.MongoDb/Resources/FreeStringMongoIdentifiable.cs	100.00% <100.00%> (ø)

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