Add suport for gists

[lburgazzoli]

Fixes #1740

Release Note

integrations stored on gist can be referenced from the cli:

  kamel run --dev https://gist.github.com/$i{user-id}/${gist-id}
  kamel run --dev gist:${gist-id}

if the environment variable `GITHUB_TOKEN` is detected, them it
will be used for GitHub authentication.

[lburgazzoli]

@nicolaferraro the tests for gists reference a gist on my account as there's no way to create organization's gists

[nicolaferraro]

@nicolaferraro the tests for gists reference a gist on my account as there's no way to create organization's gists

Sounds good