Avoid network access in build_path

[ckunki]

Currently, function build_path()

• calls _create_onprem_bucket()
• which calls buckets = service.buckets in order to verify if the requested bucket exists.
• Which requires network access.

bucketfs-python/exasol/bucketfs/_path.py

Line 517 in 3382d65

def build_path(**kwargs) -> PathLike:

bucketfs-python/exasol/bucketfs/_path.py

Line 486 in 3382d65

buckets = service.buckets

However, the user might not expect network access when only building an intentional BucketFS location.
Additionally, maybe the BucketFS to access is not available at this moment.

The current ticket therefore requests, to

• verify the bucket to exist only later on, when actually accessing the bucket.
• Optionally add a separate function for checking the validity