resurrect pgraster community module

[groldan]

The removal of GeTools ImageMosaic-JDBC left the pgraster geoserver community module orphan of its gt-imagemosaic-jdbc dependency and hence it wouldn't work anymore.

This PR brings pgraster back, by incorporating the GeoTools code that was on gt-imagemosaic-jdbc, with package refactoring as org.geotools.gce.pgraster and code trimming to only support the postgis-raster extension, instead of all the jdbc extensions it used to.

Checklist

• [x] I have read the contribution guidelines.
• [ ] I have sent a Contribution Licence Agreement (not required for small changes, e.g., fixing typos in documentation).
• [ ] First PR targets the main branch (backports managed later; ignore for branch specific issues).
• [ ] All the build checks are green (see automated QA checks).

For core and extension modules:

• [ ] New unit tests have been added covering the changes.
• [ ] Documentation has been updated (if change is visible to end users).
• [ ] The REST API docs have been updated (when changing configuration objects or the REST controllers).
• [ ] There is an issue in the GeoServer Jira (except for changes that do not affect administrators or end users in any way).
• [ ] Commit message(s) must be in the form [GEOS-XYZWV] Title of the Jira ticket.
• [ ] Bug fixes and small new features are presented as a single commit.
• [ ] Each commit has a single objective (if there are multiple commits, each has a separate JIRA ticket describing its goal).