feat: allow reuse of glab in other CLI contexts

[addshore]

Description
Moves glinstance into pkg for use by other projects.
Uses OverridableDefault hostname in auth login command.

Related Issue
Resolves #843

How Has This Been Tested?

• Tested the build which still works.
• Tested the code change included in the mwcli.

Screenshots (if appropriate):

Usage in the other CLI can be found in https://phabricator.wikimedia.org/T288373#7376987
Code change for the other CLI can be found at https://gitlab.wikimedia.org/releng/cli/-/merge_requests/47

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation
• Chore (Related to CI or Packaging to platforms)

[codecov]

Codecov Report

Merging #845 (d5b819b) into trunk (c608850) will increase coverage by 0.00%.
The diff coverage is 66.66%.

@@           Coverage Diff           @@
##            trunk     #845   +/-   ##
=======================================
  Coverage   59.41%   59.42%           
=======================================
  Files          93       93           
  Lines        7365     7366    +1     
=======================================
+ Hits         4376     4377    +1     
  Misses       2607     2607           
  Partials      382      382           

Impacted Files	Coverage Δ
cmd/glab/main.go	44.76% <ø> (ø)
commands/api/api.go	76.08% <ø> (ø)
commands/auth/status/status.go	91.86% <ø> (ø)
commands/cmdutils/factory.go	0.00% <ø> (ø)
commands/cmdutils/remote_resolver.go	58.33% <ø> (ø)
commands/config/config.go	39.02% <ø> (ø)
commands/project/clone/repo_clone.go	63.57% <ø> (ø)
internal/config/config_type.go	69.45% <ø> (ø)
internal/glrepo/repo.go	85.12% <ø> (ø)
pkg/glinstance/host.go	92.45% <ø> (ø)
... and 1 more

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[addshore]

Thanks!
I'll add this to the CLI fully for the next release and upstream any other tweaks that might make sense for this kind of usecase!