Add support for HTTP headers in Spring Cloud Config Client extension

[briansterle]

Description
The feature request is to add a new configuration property to support generic HTTP headers that would be used on the HTTP requests from the Spring Cloud config client to the Spring config server. This could be used to pass JWT credentials in an Authorization header, for example.

This is currently part of Spring's own config client implementation, accessible through the following property:
spring.cloud.config.headers

Implementation ideas
In Quarkus, the property name could be:
quarkus.spring-cloud-config.headers

taking a Map<String, String> type, as in the Spring implementation.

[quarkusbot]

/cc @geoand