Python JSON parser: Ignore invalid enum string values if ignore_unknown_fields is set (#15887)

# Motivation

This PR fixes failing conformance tests for python with name `IgnoreUnknownEnumStringValue`.

The JSON parsing spec was discussed in https://github.com/protocolbuffers/protobuf/issues/7392.

Recent equivalent changes for other languages:
* Swift: https://github.com/apple/swift-protobuf/pull/1345
* C#: https://github.com/protocolbuffers/protobuf/pull/15758

# Changes

- 1st commit is a noop  refactoring to make relevant _ConvertScalarFieldValue invocations localized
- 2nd commit introduces the child exception of `ParseError` named `EnumStringValueParseError` which is suppressed if `ignore_unknown_fields` is set
- 3rd commit updates the conformance test failure lists

Closes #15887

COPYBARA_INTEGRATE_REVIEW=https://github.com/protocolbuffers/protobuf/pull/15887 from noom:anton/7392/fix-python-test fbcc93a232a8e7c858b7cb62c1eaef1c6a54fe20
PiperOrigin-RevId: 619288323