[MGPG-135] Support Overriding / Enhance the signer in AbstractGpgMojo

[laeubi]

Currently AbstractGpgMojo has a fixed set of supported signers (and how they are constructed), it would be good to allow extensions (e.g. Tycho is also using AbstractGpgMojo) to possibly override that aspect.

This extracts the creation of the signer into a (protected) method so different types or alternative implementations can be used.

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