Is your feature request related to a problem? Please describe

Creating this issue to add a public getter for the excludes list in SourceFieldMapper.java.

https://github.com/opensearch-project/OpenSearch/blob/main/server/src/main/java/org/opensearch/index/mapper/SourceFieldMapper.java

This is required for a knn derived source path:
opensearch-project/k-NN#3006

Currently there is no efficient way to retrieve the excludes from a SourceFieldMapper object.

Describe the solution you'd like

Ask is to add a public getter function for the excludes of a SourceFieldMapper class.

Describe alternatives you've considered

The only other alternative possible in the knn derived source path is to get the string source from mapperservice, and find excludes through there, but this is not efficient.