Figure out how to cache & load LD-Contexts

[decentralgabe]

LD Contexts should be loaded and cached without making web requests. Web servers go down, and when they do, they shouldn't stop us from verifying data. I'm not sure if this is a capability of https://github.com/piprate/json-gold

[decentralgabe]

Looks like we should be able to inline replace known context URIs with full context documents https://github.com/piprate/json-gold/blob/master/ld/context.go#L116

[georgepadayatti]

Also, it adds an additional latency if there is no cache support. Facing the same problem in a project that I am working.
https://github.com/decentralised-dataexchange/acapy-mydata-did-protocol

[singyiu]

PR pending review.

json-gold does have the caching document loader implemented (RFC7324)
It is enabled by passing the caching loader into the ldProcessor options
Added it as the default option here.

Json-ld context can be loaded as a json-ld document (type map[string]interface{}) using the loadDocument function test example
or further converting it into a json-gold.Context object test example
A helper function "GetContextFromMap" was added.

[decentralgabe]

Fixed in #102