spin new -E and spin plugins -E

[itowlson]

Fixes #3494

This adds:

• spin new -E - looks for the template in the repo pointed to by the given environment
• spin plugins install -E - install the plugins suggested in the given environment, or the environment for the current app if -E with no env name
• spin plugins list -E - list the plugins suggested in the given environment (again default to current app)

Testing things I have to remove:

• The fake local env file Done
• Interpreting the -E as a file ref instead of a catalogue ref Now defaults to catalogue ref. Use https: prefix to interpret as URL, file: prefix for file path

Need:

• A way for env authors to test against a local file or a GH fork -E file:foo.toml / -E https://example.com/foo.toml
• A way to view and install the env plugins if you don't want to do so at spin new time Done, at least I think it's done, my brain is fried
• Define --accept-defaults behaviour when plugins are in play --accept-defaults will NOT suppress the plugin install prompt
• @kate-goldenring has reported seeing it offering to install already-installed plugins but I cannot reproduce this (and cannot see a code path that could open it up)

[itowlson]

Sorry I thought I had clippied this. I have stuff going on which mean I have uncommitted stuff in my working copy, so I will fix the lints in the morning along with those.

[itowlson]

For my testing I have used https://raw.githubusercontent.com/itowlson/spin/4f12a8b60dd5d66ed79df89e50fddf00f18519c3/target-envs/env-mc-envface.toml (note this is removed from the PR so you will need to address it at that specific commit).

[fibonacci1729]

Thanks for this! Works great for me.