fix: use serialisation helper function

[dhrudevalia]

Issue

When calling the GDS read functions we are not using the lazy serialisation client.
This is required for the BigQueryConnector which uses this library instead of neo4j-arrow.
Follow-up from: #804

Fix

Use self._client() instead of the _flight_client directly so that it can be lazy initialised if needed.

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[FlorentinD]

Checking on other usage, I think we also need to fix some other places:

• __exit__ should just call `self.close()
• close() .. only close the client if its not None

• graph-data-science-client/graphdatascience/query_runner/gds_arrow_client.py

  Line 143 in c774985

  self._flight_client.authenticate_basic_token(self._auth[0], self._auth[1])

  .. should also use the helper function to fetch the token?

[FlorentinD]

makes sense.

[dhrudevalia]

Checking on other usage, I think we also need to fix some other places:

• __exit__ should just call `self.close()
• close() .. only close the client if its not None

• graph-data-science-client/graphdatascience/query_runner/gds_arrow_client.py

  Line 143 in c774985

  self._flight_client.authenticate_basic_token(self._auth[0], self._auth[1])

  
  .. should also use the helper function to fetch the token?

Yep good shout. I've updated the token fetching + close method as suggested.