Support Custom Toolchains

[johnbute]

I came across a case where I had a local toolchain (.tar.gz format) generated from a CI build. However, it took me a moment to realize that swiftly does not support the use of custom toolchains.

I propose adding a 4th Toolchain version: custom()
which supports installation from local .tar.gz or .pkg files, and maybe from url (https://ci.swift.org/job/swift-PR-toolchain-macos/..../artifact/branch-main/swift-PR-....-osx.tar.gz).

Alongside a couple refactoring changes and updating methods to handle custom toolchain selectors, users would be able to install custom toolchains just like any stable or snapshot release:

swiftly install <path-of-custom-toolchain> --name <name-of-toolchain>
or
swiftly install <url-of-custom-toolchain> --name <name-of-toolchain>

Afterwards when listing toolchains, users will be able to tell the difference between custom and non-custom toolchains

`swiftly list

Swift 6.2.0
Swift 6.1.0
Swift foo (custom)
`

Finally, users can use this toolchain just like any other toolchain version:

swiftly use foo-toolchain

Wondering what people think about this, open to feedback, suggestions, and comments.

[johnbute]

Note that this was mentioned before in #101.