Add support for multiple runners

[hajapy]

Here's a proposal for adding support for launching multiple runners. The docs are not updated and the input/output name changes may be undesired, but the gist of the feature is covered. If this approach looks good I can work to clean it up. It could definitely use some more testing (I tried it with 2 runners so far).

Closes https://github.com/machulav/ec2-github-runner/issues/8

[jpalomaki]

@hajapy Out of curiosity: have you investigated if it'd be possible to use GitHub's job matrix strategy (which'd just invoke this action several times) to spawn multiple runners?

[hajapy]

@hajapy Out of curiosity: have you investigated if it'd be possible to use GitHub's job matrix strategy (which'd just invoke this action several times) to spawn multiple runners?

@jpalomaki I am not sure how getting outputs works in the context of a matrix. Regardless, it seems inefficient to run the action multiple times when the underlying aws api call supports the desired functionality directly.