Add apt repo definition option to fetch keyID directly

[philclifford]

Where a key is not otherwise readily available but the signing ID is we could maybe support direct

${ELEVATE} gpg --no-default-keyring --keyring /usr/share/keyrings/${APP}-archive-keyring.gpg --keyserver keyserver.ubuntu.com --recv ${APT_KEY_ID}

Originally posted by @philclifford in #731 (comment)

[KlfJoat]

I cobbled together my own deb-get-like system over the last 4 years in my ignorance of this project. And this functionality is the main piece that deb-get is missing compared to mine.

If this gets implemented, I'll switch to deb-get and add my software that's not already included.

[philclifford]

@KlfJoat keep an eye on the status of PR 741 :-)