http: added OVERWRITE_IF_EXISTS header manipulator (#27923)

This is to address a use case when a header should be modified only when it exists and should not be added when it does not exist.

Risk Level: Low
Testing: Unit tests.
Docs Changes: Yes.
Release Notes: Yes
Platform Specific Features: No

Fixes #27907

Signed-off-by: Christoph Pakulski <christoph@tetrate.io>

Mirrored from https://github.com/envoyproxy/envoy @ d9ba9d17016296c50069584905dee1a19427d42e

envoy/config/core/v3/base.proto

@@ -359,6 +359,10 @@ message HeaderValueOption {
     // the header already exists. If the header doesn't exist then this will add the header
     // with specified key and value.
     OVERWRITE_IF_EXISTS_OR_ADD = 2;
+
+    // This action will overwrite the specified value by discarding any existing values if
+    // the header already exists. If the header doesn't exist then this will be no-op.
+    OVERWRITE_IF_EXISTS = 3;
   }
 
   // Header name/value pair that this option applies to.