remote-add: Store all .flatpakrepo keys

Application installers want to have access to
these fields to present repositories to the user,
so store them in our repo config, using xa.
prefixed keys.

Closes: #2812
Approved by: alexlarsson
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f