Support credential download and add fields to Credential

[saberlynx]

What:

Support argument credential_format in single Credential query and include fields in the Credential entity to support download of credentialPackage, publicKey and certificate formats.

Why:

In GSA it is possible to download these formats and without the credential_format argument, this does not work.

How:

Unit tests.

Checklist:

• Tests
• CHANGELOG Entry
• Documentation

[codecov]

Codecov Report

Merging #66 (ffa67ee) into master (aff4cce) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #66   +/-   ##
=======================================
  Coverage   97.89%   97.90%           
=======================================
  Files         122      122           
  Lines        7617     7630   +13     
=======================================
+ Hits         7457     7470   +13     
  Misses        160      160           

Impacted Files	Coverage Δ
selene/schema/credentials/fields.py	96.42% <100.00%> (+0.97%)	⬆️
selene/schema/credentials/queries.py	100.00% <100.00%> (ø)

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