Add support for JATS reference export

[asmecher]

Add support for JATS export of the reference list in the JATS Template Plugin (or integrated equivalent).

Approach:

With the introduction of #10692, OJS/OMP/OPS will have two forms of citations:

• Unstructured citations (either in the form of a textarea, or a line-by-line equivalent of the same)
• Structured citations (introduced by Structured Citations #10692)

Both forms of citations are supported in JATS.

Now that the JATS Template plugin has been added as a submodule (starting with OJS 3.5.0), code should be added to that plugin to allow for citation data can be exported. Structured citations should be preferred, when available; if not, unstructured data should be used. (It may be necessary to introduce a per-submission setting to determine which is preferred.)