Use micrometer context propagation for virtual thread executor

[paulbakker]

Pull Request type

• Bugfix
• [x ] Feature
• Refactoring (no functional changes, no api changes)
• Build related changes
• Other (please describe):

Changes in this PR

Set up a Micrometer ContextSnapshotFactory and use this to wrap tasks in VirtualThreadTaskExecutor. This enables context propagation using the micrometer context-propagation library.
By default it registers the SLF4j MDC propagation that comes out of the box with micrometer context. A user can provide more accessors by either registering them in META-INF/services, which is the SPI Micrometer uses, or you can override the bean definition that's now in the DGS autoconfiguration:

@Bean
    @ConditionalOnMissingBean
    open fun dgsMicrometerContextRegistry(): ContextRegistry {
        return ContextRegistry.getInstance()
            .registerThreadLocalAccessor(Slf4jThreadLocalAccessor())
    }

WIth this PR, a datafetcher that runs on a virtual thread, has context propagated, so you can for example rely on MDC.

[srinivasankavitha]

Should the micrometer dependency be pinned here? Should we add it to the BOM instead?

[paulbakker]

Should the micrometer dependency be pinned here? Should we add it to the BOM instead?

Yes, good idea. Done.
There's one place I couldn't get rid of the version because the multi-release java21Implementation syntax didn't work with a BOM version.