Add method to list keys

[diachenko-mischa]

Adds interface for the method to list keys available in the cache.

Allows client software to not maintain the journal or another metadata to
maintain a list of entries available in the database. Moreover, having
an explicit journal is still not reliable as API does not provide an interface
for maintaining transactions in order to have an atomic insert + journal update.

This API will help to implement reliable uploading strategy: in case of
upload failure payload will be stored on the disk and retried later.

In order to query payloads effectively, software needs to access the
list of available keys.

Relates-To: HERESDK-8383

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.31%. Comparing base (e2da4d1) to head (2db0b66).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1638      +/-   ##
==========================================
+ Coverage   80.26%   80.31%   +0.04%     
==========================================
  Files         349      349              
  Lines       13953    13953              
  Branches     1493     1493              
==========================================
+ Hits        11199    11205       +6     
+ Misses       2153     2149       -4     
+ Partials      601      599       -2     

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[mykolamalik]

@diachenko-mischa
Please extend the MR description with the reasoning on why such an interface is needed. The description of the HERESDK-8383 ticket doesn't contain this information.

[diachenko-mischa]

Updated MR description.