Consider field mask path validation

[timostamm]

When serializing a google.protobuf.FieldMask to JSON, and when parsing from JSON, protobuf-go verifies that field paths are valid Protobuf identifiers, but protobuf-es does not.

The C++ and Python implementations have similar logic like protobuf-go, but it's implemented quite differently. Unfortunately, the conformance suite only provides minimal tests.

Related:

• Fix validation for FieldMask (de-)serialization to/from JSON #1323
• Feature: Path and FieldMask operations #1177