Add more metadata support in episodeSaga

[eteubert]

To import all fields from an Auphonic Production, I need write support of the following fields:

Important / Blocker

• [x] duration (and a definition of the transport format, is it ms? npt? Timestamp?)
• [x] slug

Low Priority

• [ ] license and license_url (it's an expert setting enablind license settings per episode)
• [ ] tags (technically it's a post value but it's something we are currently supporting)

[alexander-heimbuch]

• duration is already part of the episode store and persisted as the iso timestamp (npt not possible since it has to be serializable)
• slug is currently not exposed in the v2 API (as well as license and tags)

[dnkbln]

• slug isn't documented but it is contained in the v2 API

[eteubert]

@alexander-heimbuch right, duration just works, not sure what confused me last time I checked

@dnkbln only thing missing was that the slug wasn't returned in get_item but I just added it

[dnkbln]

Rest API added low priority item with PullRequest #1345