Fetch the Guzzle instance from the DI container, if it exists

[imjoehaines]

Goal

This PR adds support for fetching the Guzzle instance from the DI container using the bugsnag.guzzle alias. If bugsnag.guzzle does not exist, one will be created as before

Changeset

• Fetch bugsnag.guzzle from the DI container
• Set notify endpoint if config is set; this is because we previously set this on the guzzle instance we create, but it's needed for custom guzzle instances too
• Don't set guzzle base URI via makeGuzzle; this isn't needed with the above change

The remaining changes are to tests

Testing

• Unit test proves the right guzzle instance is used
• MR tests prove a custom guzzle instance is used by adding a header in a guzzle middleware
• Also forced the unstable tests to run to check they still pass