Make GITHUB_TOKEN an action input

It's a nuance, but there shouldn't be a need to input GITHUB_TOKEN if it can be avoided.

It may also help resolve #232.

An example implementation:

# action.yml
inputs:
  githubToken:
      description: "The GitHub access token (e.g. secrets.GITHUB_TOKEN) used to ... This defaults to {{ github.token }}."
      default: "${{ github.token }}"
      required: false

// src/index.js
const githubToken = core.getInput("githubToken", {required: true})

// OR

// src/parseConfig.js
let githubToken;
if (process.env.INPUT_GITHUBTOKEN) {
  githubToken = ConfigParser.parseString(process.env.INPUT_GITHUBTOKEN)
}

// src/index.js
const client = github.getOctokit(githubToken, {
    baseUrl: githubBaseUrl
});

Start with action.yml and trace the current token handling in src/index.js or src/parseConfig.js. Check how the action resolves its token and passes it to the Octokit client; done means an explicit GITHUB_TOKEN input is optional while the resolved token still reaches the client, with the behavior related to #232 considered.