feat(http_source): add configuration for custom HTTP responses

Adds a `response_source` configuration option to the `http_server` source that accepts a VRL
program to generate a custom HTTP response for each request.

Closes: #21013

Author: stigglor

changelog.d/21013_http_server_response_program.enhancement.md

@@ -0,0 +1,3 @@
+Add the `response_source` config option to the `http_server` source, allowing a VRL program to generate a custom HTTP response. The program receives the decoded events as input (`.` is an array of event objects) and can return a string body or an object with `status`, `body`, and `headers` fields.
+
+authors: stigglor