feature: support multiple targets

[qmx]

Hi!

I added support multiple target installs on this PR - I am more than happy to accommodate changes to this PR so this gets upstreamed, just let me know.

I know backwards compatibility was broken here, wasn't sure how y'all wanted to handle it - I'm open to implementing whatever strategy is preferred.

Thank you!

[svartalf]

Hi, @qmx!

Unfortunately, as you said, this is a breaking change and I'm absolutely against merging it in a current state - it will break a lot of CI workflows.

It is my mistake not to be clear on the contributing guidelines, so I created the Contributing guide, which also mentions the backwards compatibility and why it matters, I hope that will help us in the future.

Regarding your changes, I see a few options:

1. Brainstorm the ideas for next breaking v2 version, see #16, include your change and publish it
2. Rename input back to the target and live with that irritating input name for some time
3. Do nothing and use rustup manually after invoking this Action, as in https://github.com/actions-rs/toolchain/issues/52#issuecomment-583711435

So far I'm inclined to postpone this change till the v2 (should probably check the rustup issues tracker and see if they are planning any big changes), but I'll be happy to hear any thoughts and ideas!

[qmx]

@svartalf no worries, this was really a conversation starter PR - thanks for taking the time looking at it!

[TimNN]

I just ran into this problem (not being able to specify multiple targets), because I was setting up GitHub actions for an iOS project. Two thoughts:

1. Wouldn't a backwards-compatible way of adding this be to just provide a new targets option in addition to target (and document target as deprecated). That shouldn't break existing workflows, I would think (though I'm not that familiar with Github Actions).
2. Regarding parallelism of rustup: One can add multiple targets in a single rustup command, i.e. this works: rustup target add aarch64-apple-ios x86_64-apple-ios.