Fixes #736: Add API to manage tech extensions

[nicolaferraro]

This adds a API to manage tech extensions. These are a subset of the API described in the extension doc. Other issues are tracking remaining operations.

Supported operations are:

• POST (this accepts a binary artifact and returns the JSON metadata of the extension)
• LIST
• GET
• DELETE

API are published under "/api/v1beta1". Beta api are disabled by default. They can be enabled with toggles (env var on Deployment):

FEATURES_API_V1BETA1_ENABLED=true
FEATURES_FILESTORE_ENABLED=true

I've put a toggle also for the "v1" api, but it's enabled by default.

When a binary artifact is uploaded, it's stored in "/extensions/[id]" into the filestore (same id of the entity in the jsondb). The artifact can be retrieved later (not implemented yet) by ID or GAV. A retrieval by GAV will require first a lookup into the jsondb.

Added also IT tests (and done manual testing on Openshift).

[jimmibot]

@nicolaferraro, thanks! @KurtStam, please review this.

[nicolaferraro]

This will (probably) run correctly after rebase, when #761 is merged.

[KurtStam]

@nicolaferraro #761 has been merged, can you do a rebase?

[nicolaferraro]

Should be ok now

[pure-bot[bot]]

Pull request approved by @KurtStam - applying approved label