### Problem On-demand scans for dynamic analysis of APIs will be switching over to using DAST API instead of DAST. To support this effort a new variable `DAST_API_REQUEST_HEADERS_BASE64` is needed that is compatible with the `DAST_REQUEST_HEADERS_BASE64` variable. This variable is commonly used to contain authentication secrets, by Base64 encoding the data it's output in the job console can be masked. This requirement [came up while discussing a bug](https://gitlab.com/gitlab-org/gitlab/-/issues/378325#note_1141606427) found in the current on-demand implementation. ### Proposal Add a new variable `DAST_API_REQUEST_HEADERS_BASE64` that is a base64 encoded version of `DAST_API_REQUEST_HEADERS`. Requirements: 1. [x] Variable contents are Base64 encoded, otherwise functions the same as `DAST_API_REQUEST_HEADERS` 1. [x] Providing both `DAST_API_REQUEST_HEADERS` and `DAST_API_REQUEST_HEADERS_BASE64` results in an error message 1. [x] Do not display the Base64-decoded string in the console output 1. [x] Document this new variable and it's usage