[OMP] Add support for submission keyword metadata in ONIX extract

[asmecher]

Describe the bug
The ONIX extract doesn't currently include submission keyword metadata. From DOAB:

Please use DescriptiveDetail/Subject[SubjectSchemeIdentifier=20]/SubjectHeadingText or else DescriptiveDetail/Subject[SubjectSchemeIdentifier=04]/SubjectCode

[kaitlinnewson]

@asmecher This is ready for an initial review.

I noticed that currently the subjects are being pulled from all locales and combined together, so I changed that so it's only pulling in subjects from the publication's primary locale.

• OMP 3_3_0: pkp/pkp-lib#9894 add keywords and fix locale for subjects in onix export omp#1605
• OMP 3_4_0: pkp/pkp-lib#9894 add keywords and fix locale for subjects in onix export omp#1610
• OMP main: pkp/pkp-lib#9894 add keywords and fix locale for subjects in onix export omp#1611

[asmecher]

Thanks, @kaitlinnewson, I merged the PR. But just to confirm: you found documentation about ONIX with guidance about multilingual content, right? Could you link that here?

[kaitlinnewson]

Here's the documentation on multilingual metadata in Onix.

[kaitlinnewson]

@asmecher Additional ported PRs are linked above and ready for review.

[asmecher]

All merged, thanks! Closing.