Refactor Ec-Key Gen Tool to use pairings-signatures-library

[mxtartaglia-sl]

The ec-key-gen tool (compiled in java) currently invokes arkworks without the pairings-api. The tool must be updated to use the pairings-signatures-library instead of arkworks through jni.

The tool should then remain simple and create a file based on the base64 translation of the bytes returned by the signature library.

Any upgrade in the interface is welcomed as no users are still affected by this change.
The tool added three padding bytes, which should be discarded as they are no longer necessary.