feature(dgs_corpus): util to extract signer ids

[bricksdont]

Adds helper code to extract signer identities.

At the moment I did not add this to the DGS corpus loader, it's just a standalone function. We can discuss if that's the best solution.

Example usage:

>>> get_signer_ids_from_ilex("/Users/mathiasmuller/Downloads/1177918.ilex")
{'a': ['SH-12'], 'b': ['SH-13'], 'c': ['SH-13', 'sh-mod-1', 'SH-12']}

[AmitMY]

looks good

[bricksdont]

from my side: can be shipped. (I cannot merge myself ;-)