Add STAC Client to publish to STAC Transaction API

[lukaszlacinski]

This PR to the new esgf2 branch adds support for ESGF2 STAC Transaction API. It adds new command: esgstacpub that corresponds to esgindexpub. The new command:

• uses the same command line parameter, --map-data, to get information about dataset files
• calls functions from other files to get new dataset/files metadata documents
• converts them to the STAC Item with Assets
• obtains OAuth2 access/refresh tokens from Globus Auth for the scope of the ESGF STAC Transaction API
• sends the POST request with the Item/Assets to the ESGF STAC Transaction API

All OAuth2 related settings (client_id, scope, service URLs), that the STAC Client needs to be able to request OAuth2 tokens from Globus Auth and add them with in the HTTP Authorization header to publication requests sent to the ESGF STAC Transaction API, are set in src/python/esgcet/settings.py