Refine structure of `ConstraintViolation`

[yevhenii-nadtochii]

Currently, this type has the following nuances to resolve:

1. string format and repeated string param fields should be removed in favor of TemplateString message.
2. base.FieldPath field_path is mostly used as field_name. Let's introduce an explicit field_name, and remove the remaining path prefix to field_path_prefix field.
3. repeated violation field is unnecessary. Currently, we create two violations where a single one actually occurs. If a nested message has an invalid field, a root message always creates an unnecessary violation as a wrapper. We should try to avoid this.

[yevhenii-nadtochii]

Addressed in #173, #174, #180.