Support base64 encoded passwords via DAST_API_HTTP_PASSWORD_BASE64

To make DAST API the default engine for On-demand API Scanning the monolith will pass in basic authentication values. These values will be encoded and not sent via plain text, encoding them ensures that the runner masks the values.

The API Security engine currently expects: DAST_API_HTTP_USERNAME and DAST_API_HTTP_PASSWORD

however the monolith will instead be sending DAST_API_HTTP_PASSWORD_BASE64

The API Security engine needs to be updated to accept these as per #327070 (comment 730240965)