Improve how the @context is handled, cached, and populated.

[Exulansis]

Description

All Did Documents and Verifiable Credentials created by the library should include @context entries mandated by the spec, (e.g. "https://www.w3.org/2019/did/v1" for Did Documents). Furthermore, we need to ensure we are using up to date definitions for our terms, and that no errors occur during normalization (e.g. triples being omitted).

Note: This issue was already addressed as part of issue #337 (pr #338) and eventually merged into 4.0-beta1 through #340. In order to simplify the review and feedback process, we decided it can be re-implemented (reusing existing code where possible) as a separate PR.

For reference purposes, please see the issues mentioned above, and this notion document