Installing plugins from a specific branch or tag

### Describe the feature or problem you'd like to solve

Add support to the /plugin commands for installing plugins from a specific branch or tag of a github repo

### Proposed solution

Currently there is support for installing plugins or configuring marketplaces based on git repos using the simple `owner/repo` syntax. In the case of plugins they can be installed from a particular folder as well using `owner/repo:path/to/plugin`.

It would be nice to be able to specify a branch or tag as well. I believe the marketplace spec already supports a `ref` field for specifying the branch or tag. But that is not exposed to the command and isn't relevant for the direct plugin install scenario either.

One option would be to support `@{ref}` in the syntax, so you could end up with something like `owner/repo@branch:path/to/plugin` or `owner/repo:path/to/plugin@branch`. Though those might conflict with the `plugin@marketplace` syntax, at least visually, if not syntactically.

### Example prompts or workflows

`/plugin marketplace add owner/repo@feature-branch`
`/plugin install plugin@marketplace`
or
`/plugin install owner/repo@feature-branch`

### Additional context

It looks like the spec for marketplace also supports sha for a specific commit as well. I don't specifically have that need now, but for completeness that could be a part of it.