Simplify get_object by waiting for response headers

[passaro]

S3CrtClient::get_object was originally implemented so that it would complete immediately and return a GetObjectRequest implementation (extending Stream) to retrieve body parts. Any error from the S3 request would be returned through the stream.
We recently added additional methods (get_object_metadata in #1065 and get_object_checksum in #1123) to the response that rely on the headers returned by the (first) GetObject request. The new methods required an async signature and a complicated implementation in order to account for failures and they still do not correctly report accurate error information in some cases.
With this change, we modify get_object to await for response headers before returning either an error or a GetObjectResponse (note the name change) implementation. The ergonomics of get_object are improved:

• awaiting the initial call can already return some errors (e.g. bucket/key not found),
• get_object_checksum and get_object_metadata are now sync functions.

Does this change impact existing behavior?

Yes, get_object behavior is different, get_object_checksum and get_object_metadata signatures have changed, and GetObjectRequest was renamed to GetObjectResponse.

Does this change need a changelog entry?

Yes, it requires a breaking change entry for mountpoint-s3-client.

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