fix(ingest/snowflake): fix type annotations + refactor get_connect_args

In the latest releast of types-PyOpenSSL, they removed their dep on
types-cryptography (python/typeshed#9459). This
caused mypy to start using the type annotations provided by the
cryptography package, which caused an issue in our build. Example build
failure: https://github.com/datahub-project/datahub/actions/runs/3888835554/jobs/6636552750

Author: hsheth2

metadata-ingestion/src/datahub/ingestion/source_config/sql/snowflake.py

@@ -1,5 +1,5 @@
 import logging
-from typing import Dict, Optional
+from typing import Any, Dict, Optional
 
 import pydantic
 import snowflake.connector
@@ -153,7 +153,7 @@ class BaseSnowflakeConfig(BaseTimeWindowConfig):
         default=True,
         description="If enabled, populates the snowflake view->table and table->view lineages (no view->view lineage yet). Requires appropriate grants given to the role, and include_table_lineage to be True. view->table lineage requires Snowflake Enterprise Edition or above.",
     )
-    connect_args: Optional[Dict] = pydantic.Field(
+    connect_args: Optional[Dict[str, Any]] = pydantic.Field(
         default=None,
         description="Connect args to pass to Snowflake SqlAlchemy driver",
         exclude=True,
@@ -297,28 +297,28 @@ def get_sql_alchemy_url(
             },
         )
 
+    _computed_connect_args: Optional[dict] = None
+
     def get_connect_args(self) -> dict:
         """
-        Builds connect args and updates self.connect_args so that
-        Subsequent calls to this method are efficient, i.e. do not read files again
+        Builds connect args, adding defaults and reading a private key from the file if needed.
+        Caches the results in a private instance variable to avoid reading the file multiple times.
         """
 
-        base_connect_args = {
+        if self._computed_connect_args is not None:
+            return self._computed_connect_args
+
+        connect_args: dict = {
             # Improves performance and avoids timeout errors for larger query result
             CLIENT_PREFETCH_THREADS: 10,
             CLIENT_SESSION_KEEP_ALIVE: True,
-        }
-
-        if self.connect_args is None:
-            self.connect_args = base_connect_args
-        else:
             # Let user override the default config values
-            base_connect_args.update(self.connect_args)
-            self.connect_args = base_connect_args
+            **(self.connect_args or {}),
+        }
 
         if (
-            self.authentication_type == "KEY_PAIR_AUTHENTICATOR"
-            and "private_key" not in self.connect_args.keys()
+            "private_key" not in connect_args
+            and self.authentication_type == "KEY_PAIR_AUTHENTICATOR"
         ):
             if self.private_key is not None:
                 pkey_bytes = self.private_key.replace("\\n", "\n").encode()
@@ -337,13 +337,16 @@ def get_connect_args(self) -> dict:
                 backend=default_backend(),
             )
 
-            pkb = p_key.private_bytes(
+            pkb: bytes = p_key.private_bytes(
                 encoding=serialization.Encoding.DER,
                 format=serialization.PrivateFormat.PKCS8,
                 encryption_algorithm=serialization.NoEncryption(),
             )
-            self.connect_args.update({"private_key": pkb})
-        return self.connect_args
+
+            connect_args["private_key"] = pkb
+
+        self._computed_connect_args = connect_args
+        return connect_args
 
 
 class SnowflakeConfig(BaseSnowflakeConfig, SQLAlchemyConfig):