Support arbitrary artifact names in local file artifact staging service

The local-file based artifact staging service implementation stores artifacts in a flat directory under the exact names they are given by the artifact staging request. This assumes that all artifact names are safe file names and requires staging clients to manually escape names.

Instead, the staging service should perform its own escaping/mapping transparently and allow clients to specify arbitrary artifact staging names.

Imported from Jira [BEAM-4109](https://issues.apache.org/jira/browse/BEAM-4109). Original Jira may contain additional context.
Reported by: bsidhom.