Add HsqldbJdbcIndexedSessionRepositoryCustomizer

[MFAshby]

Similar to other relational databases, this configures the session attribute create query to perform an upsert in order not to fail in the case of concurrent requests modifying the same session attributes

Fixes: #3626

[jzheaux]

Thanks for this — filling in the Hsqldb customizer is a nice gap to close alongside the existing MariaDB/MySQL/Oracle/Postgres/SqlServer/Db2 customizers.

One thing to fix before this can be merged: HsqldbJdbcIndexedSessionRepositoryCustomizerITests currently @Bean-registers a PostgreSqlJdbcIndexedSessionRepositoryCustomizer, not the HsqldbJdbcIndexedSessionRepositoryCustomizer this PR adds — looks like a copy/paste leftover from an existing Postgres test. As written, the test never actually exercises the new class. Could you update it to register the Hsqldb customizer so the test proves the upsert behavior for the class it's named after?

Since this adds new public API, it'll also need a milestone/main-track decision from a maintainer rather than a backport — happy to help route that once the test is fixed.