Support pre-loading context files for JSON ld processor

[andresuribe87]

The SDK uses a caching document loader for the JSON LD files. This was introduced in #102.

User of the SDK would like the ability to specify manually a set of contexts along with their values that they want to be preloaded. One such example is discussed in https://github.com/TBD54566975/ssi-service/pull/613#discussion_r1278105846

This issue is to track the ability of the SDK to do so.