Validate a license file is one of the allowed licenses

Use on a pull request event to make sure the repo has a valid license.

workflow "License validation flow" {
  on = "pull_request"
  resolves = ["validate license"]
}

action "validate license" {
  uses = "datreeio/validate-license-action@master"
  args = ["MIT,ISC"]
}

1. Support getting license from file
2. Support custom license file name
3. Support * allowed license
4. Support checking the source branch and not only the master