Allowing for refresh token rotation during token refresh request - #549

@sitingren either you can give this a review when you return and I can add tests and we can move forward like that, or if you would prefer I can just hand this off to you in it's current state and you can carry it to the finish line in whatever way you see fit. I'm good with either direction.

I assume there should be an API for client user to get the new refresh token?

I assume there should be an API for client user to get the new refresh token?

I just added get_current_refresh_token to connection.py.

Test:
with vertica_python.connect(**conn_info) as conn:
cur = conn.cursor()

    cur.execute(f"SELECT 'connection {x}'")
    response = cur.fetchall()
    print(response)
    refresh_token = conn.get_current_refresh_token()

Results:
[['connection 0']]
[['connection 1']]
[['connection 2']]
[['connection 3']]
[['connection 4']]

Conclusion: Able to save and use new refresh token

@DMickens Thanks! I'll merge this PR and add more improvements (including update README) in a later PR.