`RestCatalog` with sigv4 enabled instantiates a boto session per each client call

[gruuya]

Feature Request / Improvement

I've noticed that boto3 opens a new connection on each client call with rest.sigv4-enabled set to true, which is wasteful.

This happens because a boto3 session is instantiated inside the adapter's add_headers hook (which in turn is called for each HTTP call)

iceberg-python/pyiceberg/catalog/rest/__init__.py

Lines 394 to 398 in a67c559

	def add_headers(self, request: PreparedRequest, **kwargs: Any) -> None: # pylint: disable=W0613
	boto_session = boto3.Session()
	credentials = boto_session.get_credentials().get_frozen_credentials()
	region = self._properties.get(SIGV4_REGION, boto_session.region_name)
	service = self._properties.get(SIGV4_SERVICE, "execute-api")

The boto3 session should be re-used throughout the lifetime of the catalog, like the requests session, module any creds expiration/invalidation.