Add ability to extend jinja functionality with plugins

[rahearn]

Issue description / feature objectives

As a compliance engineer, I'd like to extend jinja functionality to have more flexibility in methods available when calling trestle author jinja so that I can easily complete my document-based workflows while keeping my data in trestle/OSCAL.

Initial discussion of the use case took place in CNCF slack

Caveats / Assumptions

• some additional functionality for interacting with SSP data should be added to trestle proper, because it is common and as an example implementation for plugin authors
• The plugin format should mirror existing command plugins, and be backwards-compatible with that existing functionality

Completion Criteria

• documentation for plugins has been updated
• plugin loading code discovers both new CLI commands and new Jinja Extensions
• tests and docs exist for new jinja filters included in base trestle