Bibliography API and listing functionality

[stschiff]

This PR finally adds a new Server API /bibliography as well as a new command-line option trident list --bibliography to gather bibliography information.

The feature is straight-forward in terms of usability.

In terms of implementation, I have made some additional changes to the Aeson interface specifically for the Server Communication. Specifically I looked a bit closer how Aeson encodes for Nothings in Maybe types, and found out that by default these get encoded as null in objects. This is slightly wasteful, as null-fields can also simply be omitted in a given object. When decoding into Haskell types again, we anyway use the operator .:?, which takes care of interpreting missing keys in objects as Nothings. I have therefore added a small function called removeNulls in ServerClient.hs to remove null-fields from objects. Not sure how big the effect is, but it was simple enough and I'm glad I now understand Maybe-encoding in Aeson better. The newest version of Aeson (>2.2.2.0) actually adds an encoding operator to remove null-values, which will eventually make my function redundant, but that latest version isn't even in the latest LTS, so it'll be some time.

Running the new bibliography function on the Community-Archive reveals some gaps in our bibliography records, which we can easily fix with a bit of curation work.

[codecov]

Codecov Report

Attention: Patch coverage is 43.97163% with 79 lines in your changes missing coverage. Please review.

Project coverage is 59.24%. Comparing base (6d6e8f3) to head (2996e6d).
Report is 25 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Poseidon/CLI/List.hs	36.17%	26 Missing and 4 partials ⚠️
src/Poseidon/ServerClient.hs	50.00%	26 Missing ⚠️
src/Poseidon/CLI/Serve.hs	21.42%	10 Missing and 1 partial ⚠️
src/Poseidon/CLI/OptparseApplicativeParsers.hs	0.00%	8 Missing ⚠️
src/Poseidon/Package.hs	80.00%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #320      +/-   ##
==========================================
- Coverage   59.88%   59.24%   -0.64%     
==========================================
  Files          29       29              
  Lines        4170     4296     +126     
  Branches      481      487       +6     
==========================================
+ Hits         2497     2545      +48     
- Misses       1192     1264      +72     
- Partials      481      487       +6     

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[stschiff]

I bumped the version to 1.5.9.0, so I expect this to be merged in after #318.