Add support for automatic context-propagation with Micrometer

[sjohnr]

ThreadLocalAccessor support to be added for:

• SecurityContextHolder
• ReactiveSecurityContextHolder
• ServerWebExchange (Context attribute set by ServerWebExchangeReactorContextWebFilter)

Related gh-13733

[sjohnr]

Note that this enables DefaultReactiveOAuth2AuthorizedClientManager (and ServerOAuth2AuthorizedClientExchangeFilterFunction for WebClient) to be used with blocking execution when automatic context propagation is enabled and the context-propagation library is on the classpath.