GraphQL authentication middleware

[pedro-gutierrez]

Is your feature request related to a problem? Please describe.

We need to authenticate and authorise our GraphQL queries and mutations.

Describe the solution you'd like

A set of Phoenix plugs and Absinthe middleware in order to:

• Detect the presence of a bearer token in the authorisation HTTP header.
• Decode the token as a JWT.
• Verify the signature and expiration of that token.
• Decode and extract any claims or user information from that token and put them in the current context.

Authorisation is not in scope in this ticket.