SNOW-3020359 expand tilde to key file path in connection

[sfc-gh-dszmolka]

Please answer these questions before submitting your pull requests. Thanks!

1. What GitHub issue is this PR addressing? Make sure that there is an accompanying issue to your PR.

   This fix aims to address issue SNOW-3020359: Automatically expand tilde (~) to $HOME in private_key_file path #2746, where we do not expand tilde (~) in Connection for private_key_file configuration, thus the driver cannot read the key file for keypair auth in such cases.

2. Fill out the following pre-review checklist:

   • I am adding a new automated test(s) to verify correctness of my new code
   • I am adding new logging messages
   • I am adding a new telemetry message
   • I am modifying authorization mechanisms
   • I am adding new credentials
   • I am modifying OCSP code
   • I am adding a new dependency

3. Please describe how your code solves the related issue.

   Using pathlib.Path(path_to_key).expanduser(), which should work on both Unix and Windows bases systems, and even work without HOME envvar set (and for Linux, read it from /etc/passwd)

   Besides adding an automated test, also manually tested on Linux and Windows (latter both with \\ and / style path separator)

4. (Optional) PR for stored-proc connector:

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[sfc-gh-dszmolka]

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