Add PGP signatures and attestations to build process

[remyroy]

This PR adds 2 new elements to our build process with Github workflows:

1. It create PGP signatures based on the stored PGP private key with Github secrets. It expects 2 secrets: GPG_PRIVATE_KEY and PASSPHRASE as described with the import-gpg action. I've already added those to the eth-educators/ethstaker-deposit-cli repo. You can see the documentation on this for more details.
2. It creates build attestations to create a strong link between the workflow used to create the release assets and the eventual releases. See the documentation for more details.

[remyroy]

Fixes #31

[remyroy]

The public key used for the eth-educators/ethstaker-deposit-cli repo for now is:

-----BEGIN PGP PUBLIC KEY BLOCK-----

mDMEZjkpCRYJKwYBBAHaRw8BAQdA5vgWEja2G4GrVjZMnBIYmFrZvEnxuJ6uUS5s
adscmme0KEVEQyBTZWN1cml0eSA8ZWRjLXNlY3VyaXR5QGV0aHN0YWtlci5jYz6I
lgQTFggAPhYhBFT6BvwIYPwNzMaOPs6f8jkd8mNoBQJmOSkJAhsDBQk4ZAkABQsJ
CAcCBhUKCQgLAgQWAgMBAh4BAheAAAoJEM6f8jkd8mNoN7EBAKmmaW20VPreIvEv
QCf2CCeDqwW29MPmzAnFRBTRcqJjAQCu0Clw3k/jXJcv1mtHPpKLh5+9Nf5bsRnR
l86WZJc2Dw==
=IeL9
-----END PGP PUBLIC KEY BLOCK-----

[valefar-on-discord]

I testing this on my own branch and verified the gpg signature and the attestations without issue.

One small question: