Create WebDAV endpoint

[ericbusboom]

Create a WebDAV endpoint in the UI, running at url /webdav, which exposes all of the files in bundles to reading and writing.

This looks like the best Python library to use for the base WebDAV server: https://github.com/mar10/wsgidav

The file hierarchy is the same as the cache_keys structure: /. See Identity.cache_key

Within the directory, there is read/write access to each of the files in File records. The WebDAV interface basically just reads and writes with ambry.bundle.files.BuildSourceFile.record_to_fh and fh_to_record().

After writing, some files should be automatically synced to objects. Maybe all of them, TBD.

There could also be some virtual files. For instance, maybe all of the database records have entries, and you could read and write them as YAML or JSON. So, you could write to /webdav///bundle/tables/<table_name>/columns/<column_name>.json to change some of the properties of the column. ( Only the description, summary and similar should be editable. )