Configurable enabled keys for JML condition evaluation

This PR allows you to configure the enabled keys in the processing of the JML conditions within the JML conditional annotation texts. Currently, the set is hard coded and only contains key as the only enabled key. This means, that KeY accepts the following JML annotation texts:

/*@ ...*/
/*+key@ ...*/
/*+key+openjml@ ...*/

but ignores

/*+openjml@ ...*/
/*-key@ ...*/

Intended Change

This PR adds a new field to the GeneralSettings which contains a set of enabled keys.
You can set this field either by -PJML_ENABLED_KEYS=a,b,c on the CLI, or
via the $HOME/.key/proofIndependentSettings.json.

Currently, there is no GUI element to configure this element.

The need for this raises from case study where you want to have the same source code as the foundation, but you have different verification runs: e.g, functional behavior under overflow absence, and a second run to prove overflow absence. Now you can differentiate the specificaton between the runs. For example:

//+first@ assert x>0;
//+second@ assume x>0;

Question to the reviewer

Should there be textbox in the settings dialog?
Should there be label in the status bar stating the current enabled keys?

Type of pull request

X New feature (non-breaking change which adds functionality)
X There are changes to the (Java) code
X Changes to the settings

Codecov Report

Attention: 16 lines in your changes are missing coverage. Please review.

Comparison is base (abbe1f8) 37.98% compared to head (a8a10a8) 37.98%. Report is 15 commits behind head on main.

Files	Patch %	Lines
...java/de/uka/ilkd/key/settings/GeneralSettings.java	23.80%	16 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #3373 +/- ## ========================================= Coverage 37.98% 37.98% - Complexity 17024 17027 +3 ========================================= Files 2059 2059 Lines 126029 126051 +22 Branches 21282 21285 +3 ========================================= + Hits 47874 47883 +9 - Misses 72272 72285 +13 Partials 5883 5883 ```

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KeYProject / key