Extend helium to provide ability to fetch emails content from letters

[Burbaka]

Sometimes your API requests cause letters to be send to the GMAIL (or similar service).
These emails can contain tokens (or other information) that is required to perform the following requests in a scenario chain.
Makes sense to extend helium to implement functionality when we can query GMAIL service and fetch required info from the letter body.

Scenario example:
User registration -> Invitation letter is sent -> Fetch email content to get activation link -> Send activation request with the activation link.