[FLINK-35266][snapshot] Add E2E tests for FlinkStateSnapshot

[mateczagany]

What is the purpose of the change

Add end-to-end tests for the FlinkStateSnapshot resources.

Brief change log

• Trigger savepoint by creating a new FlinkStateSnapshot savepoint CR
• Trigger savepoint by using savepoint trigger nonce
• Trigger checkpoint by using trigger nonce
• Test periodic savepoints triggered by the operator
• Change job to upgrade mode, suspend job and assert new FlinkStateSnapshot CR created

Verifying this change

• Manually ran
• Pipeline passes

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changes to the CustomResourceDescriptors: no
• Core observer or reconciler logic that is regularly executed: no

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? not applicable

[gyfora]

This PR needs to be updated since a recent change on main