Allow omitting the standard enum prefix

[Alfus]

When StrictEnums is false, the following is now allowed:

  - oneof_enum_value: VALUE_1

The following is always allowed:

  - oneof_enum_value: PROTO2_TEST_ENUM_VALUE_1

Note that this does not impact 'encoding', which always produces protojson-compatible output.

[bufdev]

Non-strict should definitely not be the default. This makes protoyamls behavior different than protojson, and overall surprising.

[pkwarren]

Change looks good but agree this shouldn't be the default.

Do you plan to also add a plugin option in the JSON schema plugin to include shorter enum names?

[Alfus]

Every change also requires a change to the json schema plugin.
Protoyaml input has never been (and never needs to be) valid protojson. Only the output is normalized to be compatible.

[Alfus]

Honestly, because there are no backwards compatibility issues, there should be no option. This change only changes what used to be an error in to an intuitive non-error case. If any option should be supported it is the option to specify which one you want the jsonschema plugin to suggest, which is external to this library.