[SYNCOPE-1982] Improve query perfomance on OraclePlainSchemaRepoExtImpl

[valerio-crescia]

No description provided.

[ilgrosso]

@valerio-crescia thanks for this PR

Please have a look at https://github.com/apache/syncope/actions/runs/29098192671/job/86380047982?pr=1450#step:8:202 since there are some integration test failures.

Also, since I've made a few adjustments in the code, please pull before making any further change.

[ilgrosso]

@valerio-crescia the last change is too much superficial: it might let the tests succeed, but is just wrong as it barely flatten typed values to their default string representation. What could be happening with dates, for example?
Just look at other Oracle classes that are doing typesafe value comparison on queries and change the SQL query accordingly

[ilgrosso]

With some minor adjustment, the code seems to be more robust.
Thank you @valerio-crescia : FYI I've cherry-picked this change to 4_0_X and master branches.

[valerio-crescia]

You are welcome, and thanks for improving my PR