feat(sources)!: add custom auth strategy for components with HTTP server

This adds `custom` auth strategy for components with HTTP server (`http_server`, `datadog_agent`,
`opentelemetry`, `prometheus`) besides the default basic auth. This is a breaking change because
`strategy` is now required for auth - for existing configurations `strategy: "basic"` needs to be
added.

Related: vectordotdev#22213

src/sources/util/http/error.rs → src/common/http/error.rs

@@ -1,3 +1,4 @@
+#![allow(missing_docs)]
 use std::{error::Error, fmt};
 
 use serde::Serialize;

src/common/http/mod.rs

@@ -0,0 +1,12 @@
+//! Common module between modules that use HTTP
+#[cfg(all(
+    feature = "sources-utils-http-auth",
+    feature = "sources-utils-http-error"
+))]
+pub mod server_auth;
+
+#[cfg(feature = "sources-utils-http-error")]
+mod error;
+
+#[cfg(feature = "sources-utils-http-error")]
+pub use error::ErrorMessage;