Customize generated code annotations

[dmdashenkov]

Code generation annotations

In this PR we enable customization for annotations used for code generation.

If a Protobuf definition uses one of the API options (see options.proto), the resulting Java code is post-processed by adding respective API annotations.
Before this PR, the annotations were hard-coded:

• io.spine.annotation.Beta;
• io.spine.annotation.Experimental;
• io.spine.annotation.Internal;
• io.spine.annotation.SPI.

Starting from now, the annotation types may be specified with a Gradle extension:

modelCompiler {
    generateAnnotations {
        internal = 'foo.bar.MyInternal'
        beta = 'com.google.common.annotations.Beta'
        // ...
    }
}

Other significant changes:

• (SPI) field option is deleted, since there are no obvious cases for such an option;
• EnumAnnotator class is deleted, since it was never used;
• new smoke tests module annotator-tests is added in order to verify the annotator plugin behaviour in cases when custom annotations are specified.

[codecov]

Codecov Report

Merging #280 into master will decrease coverage by 0.83%.
The diff coverage is 14.63%.

@@             Coverage Diff              @@
##             master     #280      +/-   ##
============================================
- Coverage     76.12%   75.28%   -0.84%     
+ Complexity      769      760       -9     
============================================
  Files           322      324       +2     
  Lines          8494     8555      +61     
  Branches        549      552       +3     
============================================
- Hits           6466     6441      -25     
- Misses         1894     1981      +87     
+ Partials        134      133       -1

[dmdashenkov]

@armiol, PTAL.

[armiol]

@dmdashenkov LGTM with a tiny comment to address.

[armiol]

Please document the package purpose. I understand this is a test-source package, but still, let's have at least a couple of words.