feat(headers): Internal rework and introduce HTTP Headers API

Closes gravitee-io/issues#6772

pom.xml

@@ -35,7 +35,8 @@
 
     <properties>
         <gravitee-bom.version>1.0</gravitee-bom.version>
-        <gravitee-gateway-api.version>1.26.0</gravitee-gateway-api.version>
+        <gravitee-gateway-api.version>1.31.0-SNAPSHOT</gravitee-gateway-api.version>
+        <gravitee-expression-language.version>1.8.0-SNAPSHOT</gravitee-expression-language.version>
         <gravitee-policy-api.version>1.10.0</gravitee-policy-api.version>
         <gravitee-common.version>1.18.0</gravitee-common.version>
         <swagger-parser.version>2.0.14</swagger-parser.version>
@@ -72,6 +73,13 @@
             <scope>provided</scope>
         </dependency>
 
+        <dependency>
+            <groupId>io.gravitee.el</groupId>
+            <artifactId>gravitee-expression-language</artifactId>
+            <version>${gravitee-expression-language.version}</version>
+            <scope>provided</scope>
+        </dependency>
+
         <dependency>
             <groupId>io.gravitee.common</groupId>
             <artifactId>gravitee-common</artifactId>

src/test/java/io/gravitee/policy/requestvalidation/RequestValidationPolicyTest.java

@@ -15,14 +15,14 @@
  */
 package io.gravitee.policy.requestvalidation;
 
-import io.gravitee.common.http.HttpHeaders;
 import io.gravitee.common.http.HttpStatusCode;
 import io.gravitee.common.util.MultiValueMap;
 import io.gravitee.el.TemplateEngine;
 import io.gravitee.gateway.api.ExecutionContext;
 import io.gravitee.gateway.api.Request;
 import io.gravitee.gateway.api.Response;
 import io.gravitee.gateway.api.el.EvaluableRequest;
+import io.gravitee.gateway.api.http.HttpHeaders;
 import io.gravitee.policy.api.PolicyChain;
 import io.gravitee.policy.requestvalidation.configuration.RequestValidationPolicyConfiguration;
 import io.gravitee.policy.requestvalidation.validator.*;
@@ -163,7 +163,7 @@ public void shouldValidateQueryParameter_multipleRules() {
         when(request.parameters()).thenReturn(parameters);
 
         HttpHeaders headers = mock(HttpHeaders.class);
-        when(headers.get("my-header")).thenReturn(Collections.singletonList("header-value"));
+        when(headers.getAll("my-header")).thenReturn(Collections.singletonList("header-value"));
         when(request.headers()).thenReturn(headers);
 
         // Prepare template engine
@@ -272,7 +272,7 @@ public void shouldValidateTheOrderOfViolationsWhenUsingMultipleRules() {
         final String PATTERN = "^[0-9a-fA-F]{32}\\z";
 
         HttpHeaders headers = mock(HttpHeaders.class);
-        when(headers.get("my-header")).thenReturn(null);
+        when(headers.getAll("my-header")).thenReturn(null);
         when(request.headers()).thenReturn(headers);
 
         // Prepare template engine