basic_auth: support authorization header override (#36046)

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Commit Message: basic_auth: support authorization header override

Additional Description: provide a way to do basic authorization on
headers other than `:Authorization`. Say we are doing two level of basic
authorization. One at the proxy level with header name
`Proxy-Authorization` and the other one at the application level with
header name `Authorization`.

Risk Level: low
Testing: unit test
Docs Changes: changelog
Release Notes:
Platform Specific Features:
[Optional Runtime guard:]
[Optional Fixes #Issue]
[Optional Fixes commit #PR or SHA]
[Optional Deprecated:]
[Optional [API
Considerations](https://github.com/envoyproxy/envoy/blob/main/api/review_checklist.md):]

---------

Signed-off-by: Networking team <networking@lyft.com>
Co-authored-by: Networking team <networking@lyft.com>

Mirrored from https://github.com/envoyproxy/envoy @ f3ff3306f53fc0ebb6314ffad947896b56b23d29

envoy/extensions/filters/http/basic_auth/v3/basic_auth.proto

@@ -41,6 +41,12 @@ message BasicAuth {
   // If it is not specified, the username will not be forwarded.
   string forward_username_header = 2
       [(validate.rules).string = {well_known_regex: HTTP_HEADER_NAME strict: false}];
+
+  // This field specifies the request header to load the basic credential from.
+  //
+  // If it is not specified, the filter loads the credential from  the "Authorization" header.
+  string authentication_header = 3
+      [(validate.rules).string = {well_known_regex: HTTP_HEADER_NAME strict: false}];
 }
 
 // Extra settings that may be added to per-route configuration for