http: improve AWS HTTP v4 Signature auth

- Add support services without region and service prefixes in
the URL endpoint (ex. Min.IO, GCP, Yandex Cloud, Mail.Ru Cloud Solutions, etc)
by providing region and service parameters via aws-sigv4 option.
- Add [:region[:service]] suffix to aws-sigv4 option;
- Fix memory allocation errors.
- Refactor memory management.
- Use Curl_http_method instead() STRING_CUSTOMREQUEST.
- Refactor canonical headers generating.
- Remove repeated sha256_to_hex() usage.
- Add some docs fixes.
- Add some codestyle fixes.
- Add overloaded strndup() for debug - curl_dbg_strndup().
- Update tests.

Closes #6524

src/tool_operate.c

@@ -1662,7 +1662,7 @@ static CURLcode single_transfer(struct GlobalConfig *global,
           my_setopt_str(curl, CURLOPT_PROXY_SSLKEYTYPE,
                         config->proxy_key_type);
           my_setopt_str(curl, CURLOPT_AWS_SIGV4,
-                        config->aws_sigv4_provider);
+                        config->aws_sigv4);
 
           if(config->insecure_ok) {
             my_setopt(curl, CURLOPT_SSL_VERIFYPEER, 0L);