[Enhancement] Resolve the placeholders for sub-properties

[mercyblitz]

• I have searched the issues of this repository and believe that this is not a duplicate.
• I have checked the FAQ of this repository and believe that this is not a duplicate.

Details

For now on, Dubbo does not support to resolve all placeholders from property value from any PropertySource, thus it will impact Dubbo Spring Boot's externalization configuration, like this issue :

• Can provider boot features external config placeholders in properties?

What's more, Dubbo's PropertySourcesUtils will be removed and replaced by Alibaba's spring-context-support's PropertySourcesUtils

[diecui1202]

@mercyblitz How about this issue? Ready for merge?

[cvictory]

@mercyblitz My be you should commit a pr for this issue.