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_getparam.c

@@ -113,7 +113,7 @@ static const struct LongShort aliases[]= {
   {"*t", "proxy-ntlm",               ARG_BOOL},
   {"*u", "crlf",                     ARG_BOOL},
   {"*v", "stderr",                   ARG_FILENAME},
-  {"*V", "aws-sigv4",             ARG_STRING},
+  {"*V", "aws-sigv4",                ARG_STRING},
   {"*w", "interface",                ARG_STRING},
   {"*x", "krb",                      ARG_STRING},
   {"*x", "krb4",                     ARG_STRING},
@@ -806,8 +806,9 @@ ParameterError getparameter(const char *flag, /* f or -long-flag */
 
       case 'V': /* --aws-sigv4 */
         config->authtype |= CURLAUTH_AWS_SIGV4;
-        GetStr(&config->aws_sigv4_provider, nextarg);
+        GetStr(&config->aws_sigv4, nextarg);
         break;
+
       case 'v': /* --stderr */
         if(strcmp(nextarg, "-")) {
           FILE *newfile = fopen(nextarg, FOPEN_WRITETEXT);