Enable to merge defined predicates/filters with default ones while using Discovery client route locator

[ecnabogs]

Describe the solution you'd like
I would simply like to have a new boolean property, such as spring.cloud.gateway.discovery.locator.with-default-predicates-filters, enabling not having to redefine the defaults predicate(s) and filter(s) when I simply need to append such a new predicate or filter. This would largely simplify users' configuration.

Describe alternatives you've considered
The only alternative today is to follow what is mentioned in the documentation, that is not to forget to copy/paste the default configuration (path predicate and rewrite path filter as defined in GatewayDiscoveryClientAutoConfiguration).