Fix endpoint file path retrieval in get_endpoint function

[yf23]

This pull request improves the robustness of the endpoint file lookup logic in the get_endpoint function. The function now searches multiple directories for the attestation URI table file, ensuring it can be found regardless of the script's execution context.

File lookup improvements:

• Updated get_endpoint in attest.py to search for the attestation URI table file (attestation_uri_table.json or attestation_uri_table_usgov.json) in multiple directories: the directory of the current file, the directory of the script being executed, and the current working directory. If the file is not found, an exception is raised.