HDDS-12303. Move ozone.om.user.max.volume into OmConfig

[octachoron]

What changes were proposed in this pull request?

To take advantage of the type safety and reduced boilerplate offered by configuration objects (HDDS-1466), this PR moves the definition of the ozone.om.user.max.volume parameter into OmConfig, and updates its single usage accordingly.

In addition to the steps described on HDDS-12303, the following changes are also made:

• Removing the definition from ozone-default.xml, as it is now generated from the annotations.
• Removing the configuration key and default from OMConfigKeys, as there are no usages left.
• Removing the now unused imports from OzoneManager.
• Adding the full configuration key to OmConfig to allow keeping the validation unchanged in a straightforward way.
• Including the new member in copy() and setFrom(), with the rationale that if they are offered as such, they should be complete. (Even though they seem to be intended for use in specific tests that make no changes to the parameter.)
• Adding a setter for the new member to allow updating TestOmConfig.testSetFrom().
• Updating the assertion corresponding to said update.
• Adding a unit test to cover the validation moved to OmConfig.

What is the link to the Apache JIRA

https://issues.apache.org/jira/browse/HDDS-12303

How was this patch tested?

• Manually: Starting an OM in IntelliJ, and checking the value and source returned by its /conf endpoint. This was done before and after removing the parameter from ozone-default.xml, and after specifying a non-default value in ozone-site.xml.
• New and updated unit tests as described above.
• CI on fork: https://github.com/octachoron/ozone/actions/runs/13849087215

[adoroszlai]

Thanks @octachoron for the patch.