add spring starter r2dbc support

[zeitlinger]

Fixes #r2dbc

Actual changes: https://github.com/zeitlinger/opentelemetry-java-instrumentation/compare/spring-testing-project...zeitlinger:opentelemetry-java-instrumentation:spring-r2dbc

Depends: https://github.com/open-telemetry/opentelemetry-java-instrumentation/pull/11256

[zeitlinger]

• R2dbcAutoConfiguration this thing is trying to configure r2dbc if it finds ConnectionFactory and classpath:META-INF/services/io.r2dbc.spi.ConnectionFactoryProvider
• both is included in our r2dbc instrumentation library, via r2dbc proxy, shaded: https://github.com/open-telemetry/opentelemetry-java-instrumentation/blob/a6bc3b197d44937c284dcb5a7e7553af1530d639/instrumentation/r2dbc-1.0/library-instrumentation-shaded/build.gradle.kts#L22-L25
• we don't need R2dbcAutoConfiguration - but I found no way to exclude it

Options

• the user has to include the r2dbc instrumentation library manually
• find a way to disable R2dbcAutoConfiguration - maybe exclude io.r2dbc.spi.ConnectionFactoryProvider

[laurit]

@zeitlinger could you rebase so this could be merged