Add EVP proxy mode support

[vitor-de-araujo]

This PR adds a BackendConnectorSetup class, with a subclass for each backend connection mode (agentless and EVP proxy mode). These classes hold the information necessary to create new connections in a given setup (such as API key, agent URL, whether gzip is supported).

On startup, BackendConnectorSetup.detect_setup() is called to detect, based on environment variables, which backend connection mode to use, and returns an instance of the appropriate subclass. Other parts of the code that need to talk to the backend (such as the API client and the writers) receive this instance and can use it to create new connections. In this way, the rest of the code does not have to care about which mode is being used.