Add: Verify signature of sha256sums file

[jjnicola]

What:
Verify signature of sha256sums file
Jira: SC-920

Why:

How:
Set the env variable GNUPGHOME with the path to the keyring
If you launch nasl-cli, run
nasl-cli feed update -x

If you launch openvasd, run
openvasd --feed-signature-check
or add the following to the openvas.toml configuration file

[feed]
path = "/home/jnicola/install/var/lib/openvas/plugins/"
signature_check = true

Checklist:

• Tests
• PR merge commit message adjusted