Add base headers in properties to signer_headers

[tom-s-powell]

Include the headers defined in properties in the signer_headers. This is consistent with the behaviour of the default Java AWS signer: https://github.com/apache/iceberg/blob/e406e3db80d1735fe6c840ce99ecc300ae1dd763/aws/src/main/java/org/apache/iceberg/aws/s3/signer/S3V4RestSignerClient.java#L345. The headers in the auth session include the "base headers" which are those obtained from the catalog properties.

[Fokko]

Thanks for working on this @tom-s-powell, could you also add this to the documentation: https://github.com/apache/iceberg-python/blob/main/mkdocs/docs/configuration.md#rest-catalog?

[Fokko]

nit: maybe pull the len() out of the loop for performance reasons.