functions provider SPI

[erdos]

Reimplements #138

and resolves conflicts with code changed in the meanwhile. We aim to keep the code simpler this time:

• no function priorities - to keep code simple. We do not allow overriding of functions (by same name) but instead just throw an exception when function with same name already exists.
• small footprint in code diff (ideally below 100 LOC)
• automatic configuration generation (in META-INF)

Scenarios to manually test:

• java functions available in clj unit tests
• available in standalone mode (uberjar)
• available when stencil is used as a library (jar)
• available in standalone mode compiled with GraalVM native-image
• available when ran as docker service
• available when ran in native docker service

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.78%. Comparing base (0816a6c) to head (70d0cab).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #166   +/-   ##
=======================================
  Coverage   93.78%   93.78%           
=======================================
  Files          30       30           
  Lines        2076     2076           
  Branches       92       92           
=======================================
  Hits         1947     1947           
  Misses         37       37           
  Partials       92       92           

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