Make rules-based codegen validation handle new placeholders in error messages

[yevhenii-nadtochii]

Recently, we have upgraded the format of error messages. Now, these messages may contain Kotlin-style placeholders, which are substituted with the actual values when the error is composed.

Now, we need to modify our rules-based codegen to handle these placeholders. Code generators (or separate entity) should generate code, which assembles instances of TemplateString with template string itself and its placeholder values.

Some considerations upon this topic can be found in this Google Sheet.

Make sure the test coverage of error messages is solid. Now, we barely test them. Except for (set_once) option, so take a look on its SetOnceErrorMessageITest for example.

Depends on #177.
Relates with #167.