Bringing in service account string capability

[srijan-chaudhuri]

Description:

This PR adds the ability to provide credentials via an environment variable for use cases where adding a .json to the filesystem in the runtime environment is not feasible for security reasons.

Testing:

Called get_credentials in oauth2.py by declaring a "json_key_string" (item) with personal credentials in config_data (dictionary)

The result was that we received as a valid credentials object.

References:

Would highly recommend seeing this as similar approach has been executed in big query api library!

BQ: https://github.com/anelendata/tap-bigquery/blob/master/tap_bigquery/sync_bigquery.py
Implementation: https://gitlab.1password.io/data/data-issues/-/issues/3347#note_3160542

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[srijan-chaudhuri]

@dmosorast, @dsprayberry, @Vi6hal, @bryantgray would love your take on this PR, thanks!