Fix for IDENTITY-2925

Attached fix provides support to add custom grant types by allowing to add grant type validators. Previously only default validators were supported which blocked any other grant type that pass by.
In order to configure custom grant types we have to add below under /Server/OAuth/SupportedGrantTypes/ in <CARBON_SERVER>/repository/conf/identity.xml
<SupportedGrantType>
<GrantTypeName>...</GrantTypeName>
<GrantTypeHandlerImplClass>...</GrantTypeHandlerImplClass>
<GrantTypeValidatorImplClass>...</GrantTypeValidatorImplClass>
</SupportedGrantType>
Specify the grant type name under <GrantTypeName> element, grant type handler implementation of org.wso2.carbon.identity.oauth2.token.handlers.grant.AuthorizationGrantHandler under <GrantTypeHandlerImplClass> and grant type validator implementation of org.apache.amber.oauth2.common.validators.OAuthValidator under <GrantTypeValidatorImplClass> element.