SpringBoot Add a customizer for ProxyConnectionFactory.Builder from r2dbc-proxy

With upgrading to r2dbc-proxy 1.1.5.RELEASE(should be addressed by the dependency upgrade commits), this could be a change mentioned in https://github.com/spring-projects/spring-boot/issues/40010

NOTE:
Now the R2dbcObservationAutoConfiguration is more generic to r2dbc-proxy. It could be renamed to R2dbcProxyAutoConfiguration.

Comment From: mhalbritter

Thanks Tadaya! I've split the auto-configurations in two: one for the r2dbc proxy, and left the observation bits in R2dbcObservationAutoConfiguration.