[Task] Embrace cryptographic agility

## Description
Allow easy exchange and addition of different cryptographic schemes for verifying DID documents and Credentials.

## Motivation
Providing the appropriate extension points helps implementers to tailor the library to their needs and helps us staying agile regarding new developments in the identity ecosystem or broader cryptographic sphere.

## Change checklist
Add an `x` to the boxes that are relevant to your changes, and delete any items that are not.

- [ ] The feature or fix is implemented in Rust and across all bindings whereas possible.
- [ ] The feature or fix has sufficient testing coverage
- [ ] All tests and examples build and run locally as expected
- [ ] Every piece of code has been document according to the documentation guidelines.
- [ ] If conceptual documentation (mdbook) and examples highlighting the feature exist, they are properly updated.
- [ ] If the feature is not currently documented, a documentation task Issue has been opened to address this.