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

docs/cmdline-opts/aws-sigv4.d

@@ -1,5 +1,5 @@
 Long: aws-sigv4
-Arg: <provider1[:provider2]>
+Arg: <provider1[:provider2[:region[:service]]]>
 Help: Use AWS V4 signature authentication
 Category: auth http
 Added: 7.75.0
@@ -8,3 +8,10 @@ Use AWS V4 signature authentication in the transfer.
 
 The provider argument is a string that is used by the algorithm when creating
 outgoing authentication headers.
+
+The region argument is a string that points to a geographic area of
+a resources collection (region-code) when the region name is omitted from
+the endpoint.
+
+The service argument is a string that points to a function provided by a cloud
+(service-code) when the service name is omitted from the endpoint.