At present, RestClientBuilderCustomizer allows general customization of RestClientBuilder.
This is troublesome for users that want to customize `HttpAsyncClientBuilder` and
`RequestConfig.Builder` since those are set on the `RestClientBuilder`. By customizing
those two builders user lose out on Spring Boot's support for binding username, password,
connection-timeout and read-timeout properties from `"spring.elasticsearch.rest"` namespace.

This commit enhances the `RestClientBuilderCustomizer` with support for customizing
`HttpAsyncClientBuilder` and `RequestConfig.Builder` by providing additional `customize`
methods that accept the aforementioned builders. Both new methods are optional as they have
no-op default implementations.

See gh-20994