Consider how to support gPRC service config

[philwebb]

Spring gRPC 1.0 supports service config via the spring.grpc.client.channels.<name>.service-config property which is bound to a Map and ultimately passed to the client defaultServiceConfig. There’s also some code in ConfigurationPropertiesMapUtils.convertIntegerKeyedMapsToLists that will coerce the values into a supported format.

Binding to a general purpose Map has caused issues in the past so we should consider if we want to offer a strongly typed object based on https://github.com/grpc/grpc-proto/blob/master/grpc/service_config/service_config.proto.

We might need #48920 before we implement this.