Make generated messages implement `ValidatableMessage`

[alexander-yevsyukov]

This PR teaches Validation to make generated code of message types to implement ValidatableMessage interface.

This interface used to be named MessageWithConstraints and had validate() method which returned a list of ValidationConstraint. Now it returns Optional<ValidationError> as in the recently generated code.

Other notable changes:

• The code which used to work with List<String> now uses the Text type recently introduced in base.
• The code was refactored to introduce abstract class BuilderInsertionPoint.
• The descendants of this class named BuildMethodReturnTypeAnnotation and BuildPartialMethodReturnTypeAnnotation were also introduced, but could not be fully implemented because of this limitation in ProtoData.
• The JavaValidationRenderer was refactored extracting method objects to make the whole process easier to understand.
• The Validate class was renamed to ValidateBeforeReturn to better reflect the nature of the insertion point.

[codecov[bot]]

Codecov Report

Merging #54 (233cddb) into master (6edc575) will decrease coverage by 1.13%. The diff coverage is 2.43%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #54 +/- ## ============================================ - Coverage 45.03% 43.89% -1.14% - Complexity 450 451 +1 ============================================ Files 117 122 +5 Lines 2578 2654 +76 Branches 210 211 +1 ============================================ + Hits 1161 1165 +4 - Misses 1370 1443 +73 + Partials 47 46 -1 ```

[alexander-yevsyukov]

@armiol, @dmdashenkov,

In relation to the Text type, we probably need even a type with such or similar name introduced at the level of ProtoData or even as a separate small library under the package io.spine.text.

We definitely have a generic subdomain which deals with text blocks, lines of text, etc. Passing ImmutableList<String> was OK for the start, but 1) it's too many characters to read, and 2) let's not limit ourselves to generating the code only in Java.

We may also benefit from having protobuf-based type Text in other projects.

Let's discuss this when time permits.