Add JWT token input support to reana-commons

[tomondre]

Description:
Allow reana-commons to accept a JWT token in addition to the existing access token. We currently use Swagger to generate our endpoints, so this may also require reevaluating the Swagger definitions.

Checklist:

• Update API to accept either access_token or jwt
• Add basic unit test for JWT flow
• Update README with simple JWT example

Open Questions:

• Where should the JWT be checked and rotated—client side or within reana-commons?
• Do we need to rework our Swagger-generated endpoints to support JWT input?
• Should full JWT parsing/validation ocurr, and if so where?

[tomondre]

👀 reanahub/reana-commons#500