Consider implementing CapabilityAgent from webkms-client

[dlongley]

The CapabilityAgent class (or similar) may be more well suited to this library; it doesn't require any WebKMS constructs nor do WebKMS constructs directly require it. However, it is consistently and generally just used to generate an invocationSigner for signing zcaps.

https://github.com/digitalbazaar/webkms-client/blob/main/CapabilityAgent.js