Allow for external signing

[javereec]

Hi,

I was wondering if you were thinking of making it possible for external signers to be plugged in. Currently, this all happens by the library, but this strategy is not always compatible when signing needs to happen externally, for example with an HSM.

Curious to hear your thoughts on this.

[TimoGlastra]

In our fork of this library we've created an MdocContext interface which you need to provide to the library.

The context should provide callbacks for signing / hashing / verification etc: https://github.com/animo/mdoc/blob/main/src/c-mdoc.ts#L33

We needed a bit more control over the signing as well (to allow integration with external KMS), as well as have different crypto implementation for React Native (related to #3)

We might be able to contribute our changes back if there's interest in such an API, however our fork has diverged quite a bit already.

[siacomuzzi]

Great work!
Sure, all contributions are very welcome.

[warren-gallagher]

I have generated a pull request that could address this issue. #60