EclipseLink: allow reading persistence.xml from the filesystem

[adutra]

I turns out it is actually possible to read a persistence.xml file directly from the filesystem.

The trick is that it must be in a directory that will be considered as the "archive directory"; the eclipselink.persistencexml property must point to the path of the file relative to the archive directory's parent. E.g. if the persistence file is in /config/persistence.xml, the archive directory is /config and the eclipselink.persistencexml property must be config/persistence.xml.

This PR builds on that observation and expands the allowed locations for a persistence.xml file to the following ones:

1. Classpath resource
2. Classpath resource, embedded in a jar
3. Filesystem path
4. Filesystem path, embedded in a jar

Note: this PR does not modify the Helm chart, but we could look into simplifying it to use plain files instead of jars.

[eric-maynard]

To make sure I'm reading these correctly, this means the new code should work with existing config + jar setups?

[jbonofre]

Yes, it's my understanding reading the code: it's "backward" compatible, fallbacking to the "inner" persistence xml.

[adutra]

Yes, it's backwards-compatible.

[eric-maynard]

Awesome! Great find

[dimas-b]

Could it be made at build time?.. or perhaps by test code itself?

[adutra]

Certainly... it's just 1.5Kb though, is that worth the hassle?

[dimas-b]

My concern is that if EclipseLink config evolves, the jar would go out of sync with the production code... But this is not likely, I guess... I think it's ok to merge "as is",

[eric-maynard]

I do think in the future we should be making more stuff at run/test time (e.g. the python client code can be generated from the spec). But I agree that doesn't need to block this fix

[adutra]

Related: #95.