git: Enable providers with custom domains

[khayyamsaleem]

• fix(git): Add support for custom domain permalinks and provider configuration
• create settings structure for git hosting providers

[khayyamsaleem]

Targets to fix #23103 but could use some design input

[khayyamsaleem]

self-review:

• i found myself wanting to refactor the provider_registry notion into something a bit more dynamic, like a factory pattern to create providers all with the same pattern. little less hardcoding. but opted for minimal diff in the first pass to get the idea across. would prefer the shape of the static map to be type => registry_creator as opposed to name => registry_instance, and then the registry instances would only be created on-demand when a provider-specific utility is required.
• am i accessing the settings the right way? is it fine to access the global settings in the init?
• are there places to document the settings options I'm choosing to add? is there a way to have them self-documented with code-gen?
• rather than a separate top-level git_providers settings key, should this be reshaped to be a member of a git config key, with git.providers under it? though, noticed different git-related settings like git_panel already occupying separate top-level config keys, so not sure what the best pattern is here.