Publish to Maven

[sequencer]

We need to publish diplomacy to maven, which makes RocketChip and other repository being able to use this without compiling from source, I have added the publish CI infrastructure, beside this we still need to set environments below:

PGP_PASSPHRASE: ${{ secrets.PGP_PASSPHRASE }}
PGP_SECRET: ${{ secrets.PGP_SECRET }}
SONATYPE_PASSWORD: ${{ secrets.SONATYPE_PASSWORD }}
SONATYPE_USERNAME: ${{ secrets.SONATYPE_USERNAME }}

I can provide my GitHub PGP key for signing and publishing, beside this, SONATYPE_USERNAME and SONATYPE_PASSWORD should be provided too, asking @mgielda for help.