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

lib/setopt.c

@@ -641,7 +641,8 @@ CURLcode Curl_vsetopt(struct Curl_easy *data, CURLoption option, va_list param)
 
   case CURLOPT_AWS_SIGV4:
     /*
-     * String that holds file type of the SSL certificate to use
+     * String that is merged to some authentication
+     * parameters are used by the algorithm.
      */
     result = Curl_setstropt(&data->set.str[STRING_AWS_SIGV4],
                             va_arg(param, char *));