feat: mainsail release with gpg file

[Neko-vecter]

Requested feature:

Recently, I've been trying to add mirror support to Moonraker. I'm need a file that I can use for verification. I hope Mainsail can include an .asc file for GPG verification when release the next version.

Also release a public key

• mainsail.zip
• mainsail.zip.asc (gpg file)

Solves the following problem:

• moonraker mirror
• verify release

Additional information:

Relate PR Arksine/moonraker#1081

[Neko-vecter]

After I have confidence with workflow I will submit a PR.

[meteyou]

vouch