Improve proto field references, update README.md

[alexander-yevsyukov]

This PR makes the spine package explicit in Protobuf types of the validation module. This is needed to make it clear in the documentation of the Validation library that the referenced types belong to the Spine Compiler API.

Other notable changes

• The README.md file was updated with the description of recently introduced features.
• config was updated.
• Removed unnecessary nesting in TimestampRepeatedWhenSpec.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.88%. Comparing base (49d4156) to head (f551d89).
⚠️ Report is 13 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #192   +/-   ##
=========================================
  Coverage     83.88%   83.88%           
  Complexity      304      304           
=========================================
  Files            60       60           
  Lines          1086     1086           
  Branches         46       46           
=========================================
  Hits            911      911           
  Misses          165      165           
  Partials         10       10           

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