Add support for User Impersonation (#309)

Add ability to declare principle username to support User Impersonation within Presto. This allows the authentication with an user configured for supporting username assumption so that all queries run as the said user.

pyhive/presto.py

@@ -79,7 +79,7 @@ class Cursor(common.DBAPICursor):
     visible by other cursors or connections.
     """
 
-    def __init__(self, host, port='8080', username=None, catalog='hive',
+    def __init__(self, host, port='8080', username=None, principle_username=None, catalog='hive',
                  schema='default', poll_interval=1, source='pyhive', session_props=None,
                  protocol='http', password=None, requests_session=None, requests_kwargs=None,
                  KerberosRemoteServiceName=None, KerberosPrincipal=None,
@@ -89,6 +89,8 @@ def __init__(self, host, port='8080', username=None, catalog='hive',
         :param host: hostname to connect to, e.g. ``presto.example.com``
         :param port: int -- port, defaults to 8080
         :param username: string -- defaults to system user name
+        :param principle_username: string -- defaults to ``username`` argument if it exists,
+            else defaults to system user name
         :param catalog: string -- defaults to ``hive``
         :param schema: string -- defaults to ``default``
         :param poll_interval: int -- how often to ask the Presto REST interface for a progress
@@ -121,7 +123,22 @@ class will use the default requests behavior of making a new session per HTTP re
         # Config
         self._host = host
         self._port = port
-        self._username = username or getpass.getuser()
+        """
+        Presto User Impersonation: https://docs.starburstdata.com/latest/security/impersonation.html
+
+        User impersonation allows the execution of queries in Presto based on principle_username
+        argument, instead of executing the query as the account which authenticated against Presto.
+        (Usually a service account)
+
+        Allows for a service account to authenticate with Presto, and then leverage the
+        principle_username as the user Presto will execute the query as. This is required by
+        applications that leverage authentication methods like SAML, where the application has a
+        username, but not a password to still leverage user specific Presto Resource Groups and
+        Authorization rules that would not be applied when only using a shared service account.
+        This also allows auditing of who is executing a query in these environments, instead of
+        having all queryes run by the shared service account.
+        """
+        self._username = principle_username or username or getpass.getuser()
         self._catalog = catalog
         self._schema = schema
         self._arraysize = 1