Update SPIRE Arch/Design diagrams

[amoore877]

Noticed for https://github.com/spiffe/spire/blob/main/doc/images/cache_mgr.png related to work to completely change how Agent SVID caching works in #4224

The list of diagrams for SPIRE needs review, updating, and supplementing to match new realities for plugins and internal details.

Any new plugins or major refactorings going forward should probably also get diagrams added as a condition to closing out related issues? It's likely in such cases someone will have made a diagram anyway for such a proposal, so hopefully that would not be too much of an additional burden.

[stevend-uber]

Do we have a standard for how images are generated? I made this for out team that displays the high-level flow of the event-based cache engine.