Overhaul metadata merging and add 'ghcup config add-release-channel URI'

Wrt #328 (closed) @abel

This PR does two things:

1. makes merging release channels more robust (previously we would only merge versions if the tool exists in the main channel)
2. adds cli support, so to get pre-releases you would do: ghcup config add-release-channel https://raw.githubusercontent.com/haskell/ghcup-metadata/master/ghcup-prereleases-0.0.7.yaml

This might make adding pre-releases easier as described in #328 (comment 414294)

---

So a github workflow would look as follows:

jobs:
  build:
    runs-on: ${{ matrix.os }}
    strategy:
      fail-fast: false
      matrix:
        os: [ubuntu-latest, macOS-latest, windows-latest]
        ghc: ['8.10.7', '9.3.2.20220308']
        cabal: ['3.6.2.0']

    steps:
    - uses: actions/checkout@v2

    - name: enable pre-releases
      run: ghcup config add-release-channel https://raw.githubusercontent.com/haskell/ghcup-metadata/master/ghcup-prereleases-0.0.7.yaml
      shell: bash

    - uses: haskell/actions/setup@v1.2
      with:
        ghc-version: ${{ matrix.ghc }}
        cabal-version: ${{ matrix.cabal }}

Obviously, this will only work for windows if https://github.com/haskell/actions/issues/70 is merged. But since ghcup is pre-installed on ALL github environments, you can simply drop haskell/actions/setup@v1.2 and do this instead:

    - name: Install ghc/cabal
      run: |
        ghcup install ghc --set ${{ matrix.ghc }}
        ghcup install cabal ${{ matrix.cabal }}