Unix domain socket support

[vitor-de-araujo]

Add support for talking to the agent via Unix domain socket.

Add logic for deciding whether to use the Unix domain socket, based on https://github.com/DataDog/dd-trace-py/blob/4.0/ddtrace/internal/settings/_agent.py#L32 . Basically:

• If the full tracer agent URL is specified explicitly by the user, we use that;
• If the full URL is not specified, but the hostname or port is, we construct a full HTTP url from them;
• If neither is specified, and /var/run/datadog/apm.socket exists, we use that;
• Otherwise, we default to http://localhost:8126.

Previously, for EVP proxy mode, we would pass the EVP base path as part of the URL (i.e., BackendConnector(url="http://localhost:8126/evp_proxy/v4"), but this does not work for unix:// URLs because the base path in the URL is the filename. To solve this, we allow passing an explicit base_path parameter to BackendConnector (e.g., BackendConnector(url="unix:///var/run/datadog/apm.socket", base_path="/evp_proxy/v4")).