feat: Add support for alternate download locations

[DanielVoogsgerd]

This PR introduces new functionality to branectl download services

Where services used to only support GitHub releases (and dockerhub for docker images), the input is now more freeform.

One can now for example input: github:danielvoogsgerd/brane/worker-instance:nightly to download the nightly version from my personal fork.

or in case of the default version on github these will all be synonyms:

• worker-instance
• worker-instance:latest
• brane/worker-instance (or with :latest)
• braneframework/brane/worker-instance (or with :latest)
• github:braneframework/brane/worker-instance (or with :latest)
• gh:braneframework/brane/worker-instance (or with :latest)

It will assume the main repo, the latest version, and github, where a field is omited

For images it also extended the support:

We should now also support GitHub container registry (ghcr.io), so we can start publishing containers there as well.

These can be downloaded using for example: ghcr:brane-api for the brane-api image, note that these images are not yet available as of yet.

There are still some things that need to be sorted out:

• Testing of various registries and hosts
• Regexes are incomplete for the allowable charsets
• Aliases to get the old naming scheme back, e.g. aux, central, worker
• Document new functionality inside the codebase and in the manual
• Expand test suite
• Resolve warnings (mainly functino as a reminder now)