Add Support for Custom Header Configurations in RESTCatalog

[geruh]

This PR Fixes #455 by adding ability to use custom headers in PyIceberg's RESTCatalog, making it consistent with the Java implementation of the RESTCatalog. In the Java implementation, configs have the flexibility to be specified at two points: initially, through catalog properties provided at creation, and dynamically, based on the properties included in the server's response to the getConfig endpoint call. Which ensures that all the subsequent requests are made with these headers. One thing to call out is headers are also case insensitive, and this change respects that.

Catalog Properties

catalog = RestCatalog("rest", uri=TEST_URI, warehouse="s3://some-bucket", **{"header.Content-Type": "application/vnd.api+json"})

getConfig Response

{
  "defaults": {
    "header.cache-control": "no-cache"
  },
  "overrides": {}
}

Header logic in Java RESTCatalog: https://github.com/apache/iceberg/blob/4090a8860061f58748e3faa6804094f90d3575f3/core/src/main/java/org/apache/iceberg/rest/RESTSessionCatalog.java#L165

@Fokko @HonahX

[Fokko]

Looks good, can you add this to the docs as well? :)

[HonahX]

LGTM!

[HonahX]

Merging. Thanks @geruh for working on this and @Fokko for reviewing!