Use public interface for HTTP Interface documentation

Closes gh-34443

Author: sdeleuze

framework-docs/modules/ROOT/pages/integration/rest-clients.adoc

@@ -857,7 +857,7 @@ Start by creating the interface with `@HttpExchange` methods:
 
 [source,java,indent=0,subs="verbatim,quotes"]
 ----
-	interface RepositoryService {
+	public interface RepositoryService {
 
 		@GetExchange("/repos/{owner}/{repo}")
 		Repository getRepository(@PathVariable String owner, @PathVariable String repo);
@@ -908,7 +908,7 @@ For `RestTemplate`:
 [source,java,indent=0,subs="verbatim,quotes"]
 ----
 	@HttpExchange(url = "/repos/{owner}/{repo}", accept = "application/vnd.github.v3+json")
-	interface RepositoryService {
+	public interface RepositoryService {
 
 		@GetExchange
 		Repository getRepository(@PathVariable String owner, @PathVariable String repo);

framework-docs/src/main/java/org/springframework/docs/integration/resthttpinterface/customresolver/CustomHttpServiceArgumentResolver.java

@@ -29,7 +29,7 @@
 public class CustomHttpServiceArgumentResolver {
 
 	// tag::httpinterface[]
-	interface RepositoryService {
+	public interface RepositoryService {
 
 		@GetExchange("/repos/search")
 		List<Repository> searchRepository(Search search);