Did web http client

[vdods]

This adds the HTTP client as discussed in https://github.com/spruceid/ssi/issues/516

Note that this is a breaking change because of the semantics of the DIDWeb struct:

• Currently, DIDWeb is an empty struct and can be instantiated as DIDWeb by itself (no {} needed), and appears in various places as &DIDWeb.
• This change requires DIDWeb to be instantiated via methods DIDWeb::new_with_default_http_client (this is the drop-in replacement for existing functionality) or DIDWeb::new_with_http_client (if you want to specify your own HTTP client).

I altered the various call sites that were using &DIDWeb, and changed them to use an instance created with DIDWeb::new_with_default_http_client. However, codebases downstream of ssi that use DIDWeb will need to update their uses of DIDWeb (e.g. when constructing a DIDMethods) in order to integrate this change.