Provide Micrometer Context Propagation for SecurityContext

Add context propagation support via Micrometer Context Propagation for the SecurityContext, between SecurityContextHolder and ReactiveSecurityContextHolder.

This would allow applications to easily cross between the reactive <-> imperative border in either direction, and have the SecurityContext available on both sides.

Examples:

• a WebFlux application could use the handle or tap operators (which propagate from Context to ThreadLocals) to call into imperative code that expects the SecurityContext to be accessible from SecurityContextHolder.
• a Spring WebMVC application could use the captureContext operator (which propagates ThreadLocals to Context) to call into reactive code that expects the SecurityContext to be accessible from ReactiveSecurityContextHolder.

Start by reading the Micrometer Context Propagation documentation and the SecurityContextHolder and ReactiveSecurityContextHolder entry points. Use the described handle, tap, and captureContext examples to assess propagation in both directions; done means SecurityContext is available across the reactive-to-imperative and imperative-to-reactive boundaries.