header_mutation: Apply header mutations specified in all route levels (#30220)

* header mutation with all route level

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* remove dead code

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* remove dead code

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* utility function

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* fix format

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* fix format

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* fix typo

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* update header mutation test to match new impl

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* update test

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* tweak

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* fix format

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* fix

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* merge fix

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* fix format

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* add config option for specific

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* add release note

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* reword the comment

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* fix typo

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* fix format

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* test coverage

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* nit

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* reword

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* fix

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* coverage

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* comment

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* fix typo

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* remove

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* retrigger CI

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* set most_specific_wins false

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* tweak comments

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* update

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* fix format

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* update per review

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---------

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Mirrored from https://github.com/envoyproxy/envoy @ 5b7bbef9d0d5f2c569034b71e6030c29652e3ac5

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

@@ -23,8 +23,7 @@ message Mutations {
   repeated config.common.mutation_rules.v3.HeaderMutation response_mutations = 2;
 }
 
-// Per route configuration for the header mutation filter. If this is configured at multiple levels
-// (route level, virtual host level, and route table level), only the most specific one will be used.
+// Per route configuration for the header mutation filter.
 message HeaderMutationPerRoute {
   Mutations mutations = 1;
 }
@@ -33,4 +32,12 @@ message HeaderMutationPerRoute {
 // always be applied first and then the per-route mutation rules, if both are specified.
 message HeaderMutation {
   Mutations mutations = 1;
+
+  // If per route HeaderMutationPerRoute config is configured at multiple route levels, header mutations
+  // at all specified levels are evaluated. By default, the order is from most specific (i.e. route entry level)
+  // to least specific (i.e. route configuration level). Later header mutations may override earlier mutations.
+  //
+  // This order can be reversed by setting this field to true. In other words, most specific level mutation
+  // is evaluated last.
+  bool most_specific_header_mutations_wins = 2;
 }