feat: Add reviewer assign action

[toshimaru]

Description

This PR adds a feature which enables label-actions to assign reviewer(s) from the configuration.

Setting example

'review request label':
  prs:
    reviewers:
      - toshimaru
      - ...
    number-of-reviwers: 1

[acepaceNS]

@toshimaru this is great Can I use your fork of this?

[toshimaru]

@acepaceNS I've released my fork version of label-actions. You can use this :)

name: 'Label Actions'

on:
  pull_request:
    types: [labeled, unlabeled]

jobs:
  job:
    runs-on: ubuntu-latest
    steps:
      - name: Checkout
        uses: actions/checkout@v2
      - uses: toshimaru/[email protected]
        with:
          github-token: ${{ github.token }}
          process-only: 'prs'