Implement our own did:key creation/resolution

[amika-sq]

Similar to #41 and #42. There's a desire to implement did methods in our codebases, and not rely on third-party libraries that implement the current specs.

This task is to remove Spruce's did-method-key dependency (added in PR #25) from the dids crate and implement it ourselves.

[KendallWeihe]

We're currently relying on the spruce crate and mapping to/from our types. This will remain in place until it becomes an apparent blocker.

Reference #97

[KendallWeihe]

Reopening, will close once PR merged