Add path_prefix to Subscriptions API request builder

[pl-gideon]

Closes #1017

Proposed Changes:

For inclusion in changelog (if applicable):

1. Add path_prefix to Subscriptions API request builder.

Diff of User Interface

Old behavior:

New behavior:

path_prefix is an optional parameter for google_cloud_storage, amazon_s3, azure_blob_storage, and oracle_cloud_storage in subscriptions_request.

PR Checklist:

• This PR is as small and focused as possible
• If this PR includes proposed changes for inclusion in the changelog, the title of this PR summarizes those changes and is ready for inclusion in the Changelog.
• I have updated docstrings for function changes and docs in the 'docs' folder for user interface / behavior changes
• This PR does not break any examples or I have updated them

[asonnenschein]

LGTM!