Custom headers/cookies support for endpoints

[szymon-owczarzak]

Short Description

Add support for defining custom headers/cookies when defining an endpoint.

Solution (optional)

Admin can add custom headers/cookies with values when defining an endpoint.

Acceptance Criteria

• Admin can add multiple headers (with corresponding values) to endpoint configuration
• Admin cannot add second header with the same name
• Header set by admin will overrite header if it exists
• Headers are send with all requests that use this endpoint config