Fix/session guzzle client

[Cawllec]

Goal

Enables using the same GuzzleHttp\Client for all HTTP requests, meaning any custom options set will be applicable across event, session, & build notifications.

Design

• Adds a public method deliverPayload to the HttpClient
• Changes SessionTracker to use deliverPayload for session delivery
• Adds notifyEndpoint to configuration with appropriate getter & setters
• Deprecate getSessionClient method on Configuration and Client
• Extracts base_uri/base_url from passed guzzle clients and assigns the value to notifyEndpoint to retain existing behaviour.

Tests

• Updates existing tests to work with any API changes:
  • Removes reflection into HttpClient when testing default client setup
  • Guzzle client now expects blank headers when delivery build or deploy notifications
• Adds tests and lines in existing tests referencing new behaviour.
• Includes updates for tests established in SessionTracker tests #520 , requiring that to be merged first.

[imjoehaines]

Continued in #587