feat: update GitHub issues API to support fetching issues by repository host

[sonikro]

Hey, I just made a Pull Request!

Problem

1. Not all groups are (github groups), some are manually created groups and they don't have locations that are valid URLs. WHen this is the case, the application crashes when trying to parse the hostname from the URL.
2. A Group can own Github Repositories across multiple github hosts

By moving the hostname discovery from the Group to the Repo (and splitting the query into one per github host) we solve both problems.

This allows groups that are not synced from Github to still use the plugin + adds a new feature that allows you to see all of your issues across multiple github instances.

✔️ Checklist

• A changeset describing the change and affected packages. (more info)
• Added or updated documentation
• Tests for new functionality and regression tests for bug fixes
• Screenshots attached (for UI changes)
• All your commits have a Signed-off-by line in the message. (more info)

[backstage-goalie]

Changed Packages

Package Name	Package Path	Changeset Bump	Current Version
@backstage-community/plugin-github-issues	workspaces/github/plugins/github-issues	minor	v0.19.1

[backstage-goalie]

Thanks for the contribution!
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