Add environment-variable profile loader for Python client

[vinayroyt]

Summary

• add DeltaSharingProfile.from_env() to build profiles from environment variables
• add tests for default/custom env var names

Testing

• pytest python/delta_sharing/tests/test_protocol.py -k from_env
• Also ran below script to validate independently

import os

from delta_sharing.protocol import DeltaSharingProfile


def main() -> None:
    os.environ.setdefault("DSHARING_VERSION", "1")
    os.environ.setdefault("DSHARING_TOKEN", "token")
    os.environ.setdefault(
        "DSHARING_ENDPOINT",
        "https://localhost/delta-sharing/",
    )
    os.environ.setdefault("DSHARING_EXPTIME", "2021-11-12T00:12:29.0Z")

    profile = DeltaSharingProfile.from_env()
    assert profile.share_credentials_version == 1
    assert profile.bearer_token == "token"
    assert profile.endpoint == "https://localhost/delta-sharing"
    assert profile.expiration_time == "2021-11-12T00:12:29.0Z"

    print("profile from env OK")


if __name__ == "__main__":
    main()

Issue

Closes #448