feat: bubble up the entire reqwest error

[gruuya]

This PR changes the conversion of the reqwest error, that may occur in the iceberg-rest-catalog crate, from InvalidFormat to External variant.

Besides the fact that External seems a more appropriate choice semantically, there's also the matter of obfuscating relevant information buried in the error. The present "downcasting" the reqwest error into a string wrapped up by InvalidFormat results in error messages like error sending request for url (https://lakekeeper-ssl.dev/catalog/v1/568203c4-2034-11f0-894d-bb6b55e6f56f/namespaces) doesn't have the right format. This is not informative enough, but it also does not allow users of this crate to further troubleshoot the error as only its string representation remains.

In contrast, if wrapped by External, one could use a recipe like seanmonstar/reqwest#2342 (reply in thread), in order to unravel more information and present it to the user.

[JanKaul]

Makes a lot of sense. Thank you!