Add dummy sheet test

[gaurav]

This PR adds a "dummy sheet test" (#8) -- it runs sheet2linkml with a Google Sheet I've created solely for testing at https://docs.google.com/spreadsheets/d/1HpsF12vl_CeIzV2TEtANeysnh7cy6YOUDdv6fMXT00I/edit#gid=1810442705, and ensures that the output is identical to the schema generated on previous runs, which is included in this PR.

In addition to this, this PR also fixes some minor bugs found along with the way:

• The CCDH Terminology Service enumeration endpoint changed its format, so we have to modify our API to match it.
• The CCDH Terminology Service now returns 404s if the field doesn't exist or is not an enumeration, so we modify our code to catch that possibility.
• Fixes #10 by allowing the use of Google API Service Accounts in an environmental variable, allowing testing on GitHub Actions.
• Added pytest to support testing.
• Added a command line option to set the Google Sheet ID.
• Added checks to ensure that the logging_config and google_sheet_id command line arguments are set before using them.
• Made sheet2linkml explicitly set the error code to 0 in case of successful execution.
• Replaced a stray print() with logging.debug().

[gaurav]

@turbomam @jooho-lee-kim You don't need to review this PR (unless you want to!), but I just wanted to let you both know that this test now exists. Let me know if you have any ideas on improving it, either in this PR or in another one!