feat: adds Provider & GitLab support

[ZeroWiggliness]

I have added the majority of the code for adding new providers and GitLab support.

Adds a --provider flag. Defaults to github but allows gitlab.
Deprecates some github specific flags. github-release for example. This is now just release.
Adds a provider interface and makes GitHub and Gitlab derive from this.
Adds the abiltiy to set the hostUrl. This is important for being able to generate correct links. Also needed because I use GitLab enterprise so its not default.

Missing:
Backfill errors so it may not work for a mono repository.
Missing something about adding files.

Since none of my use cases needed these I am not sure what to fill in. For the most part it tries to reuse as much GitHub objects as possible since they are actually pretty generic. Lots of code could be consolidated.

To use it you can use the following in gitlab CI/CD. Dont forget to create your own GITLAB_TOKEN. I think you need the api and a couple of write permissions for issues and commits. This just pulls and builds from my branch and runs as normal:

`
build_job:
stage: build
image: node:latest
script: |
git clone -b feature/provider https://github.com/ZeroWiggliness/release-please.git release-please
cd release-please
npm install
npm run compile

node build/src/bin/release-please.js release --token=$GITLAB_TOKEN --repo-url $CI_PROJECT_PATH --target-branch $CI_COMMIT_BRANCH --provider gitlab --release-type simple
node build/src/bin/release-please.js release-pr --token=$GITLAB_TOKEN --repo-url $CI_PROJECT_PATH --target-branch $CI_COMMIT_BRANCH --provider gitlab --release-type simple

rules:
- if: $CI_COMMIT_BRANCH == "main"
`

Fixes #1021 🦕

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