feat(HttpSourceAcl): Add optional configuration header configurations

[NicolasFruyAdeo]

Add 2 optional configuration properties:

• contentlength.required: Defaults to false, fail fast rather than skipping HTTP response body length validation when set to true. Corresponding environment variable: SOURCE_HTTP_CONTENT_LENGTH_HEADER_REQUIRED
• headers: Defaults to empty, adds extra headers on the HTTP requests sent by the HttpSourceAcl, overriding default headers in case of duplicates, including those added from another configuration, authentication for example. Corresponding environment variable: SOURCE_HTTP_HEADER_OVERRIDES formatted as CSV such as "k1:v1,k2:v2"