This slightly simplifies calling code by centralizing the common 3-liner to create a JWT from claims, signed by a key.
But more importantly, it reduces the risk of `key.PreProcessToken()` being forgotten, which will become relevant in upcoming PRs:
`key.PreProcessToken()` adds the key id to the JWT header, which is important to efficiently validate tokens when multiple validation keys are supported (that is not the case yet)
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/11067
Co-authored-by: Nils Goroll <nils.goroll@uplex.de>
Co-committed-by: Nils Goroll <nils.goroll@uplex.de>
