Add support for additional HTTP headers?

[ml-evs]

As added in Materials-Consortia/optimade-python-tools#806.

Not sure if it makes sense to support this within the action (e.g. API keys via GitHub secrets), but happy to help out if we think it is worthwhile.

[CasperWA]

It seems like a good idea. To begin with it could be very raw input value, e.g., a complete JSON object as a value: headers: '{"X-Github-Header": "my header value"}'?

Later, one could add more user-friendly inputs that creates a specific header or set of headers. We'd need use cases for this though.