Add did:web resolver

[KendallWeihe]

Closes #93 and #76

This is very barebones, I opened follow-up tickets (the primary being DNS over HTTPS), but this is a good start.

I pulled DecodeID() into its own function for unit testing purposes; open to other ideas!

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Opened new issues:

• didcore.Document @context non-string types #92 (this ticket will create more tickets)
• did:web resolver offer TLS configuration #94
• did:web resolve DNS over HTTPS #95
• did:web resolve bring-your-own-client #96

[KendallWeihe]

I'm just realizing I didn't heed this advice ... I'll look at other implementations here shortly

[KendallWeihe]

Alright I reviewed the other implementations, the main thing I need to follow-up here with is DNS over HTTPS. I added a relevant comment here. This is ready for review.

[KendallWeihe]

FYI LinkedIn's DID Doc won't resolve because it has a number of mixed-type properties in it. Added support for part of that here #104