[Enhancement] Improve Shiro's Spring Support

### Search before asking

- [X] I had searched in the [issues](https://github.com/apache/shiro/issues?q=is%3Aissue) and found no similar issues.

### Enhancement Request

I think, it would be better for Shiro's Spring support to integrate with Spring Security (e.g. Spring Sec, delegate to Shiro), instead of as a replacement  This would reduce a lot of code, footprint, and complexity of the integration.

### Describe the solution you'd like

Create a Spring Security `AuthenticationProvider` that delegates to Shiro.

I've hacked on this a couple of times but I ran into a few minor issues each time, and then ran out of time to continue.

I'd love to hear other thoughts on this

### Are you willing to submit PR?

- [X] Yes I am willing to submit a PR!

Start by reviewing Shiro's existing Spring support and Spring Security's AuthenticationProvider contract. Clarify how delegation should interact with the current integration, then define tests for authentication behavior and document the compatibility and delegation behavior that constitute done.

Written by the indexing model from the issue text.