Provide a Pre-Exisiting Installation ID

[NickLiffen]

@tibdex 👋

Would you be open to a pull request that adds the capability to provide a pre-existing Installation ID? 🤔

the majority of this GitHub Action does what we need, however, we have a use case where we just need a create a JWT from a pre-installed GitHub App, vs creating a new "installation".

We could keep installationID optional? Would love your thoughts? 💯

Also open to creating a few unit tests if you're interested 👍 Something like this. I think I can make it work so it would just be a "minor" release, e.g. new feature 👍

Thanks!