Add support for merging strategies

[rahul-malik]

Currently we assume that overwrite is the write strategy for merging maps, lists, etc. Based on the application this assumption can be incorrect and the desired interaction is likely a merge of the data types instead.

This likely means augmenting how we declare properties to allow users to specify this and updating the merge with model code.

This might not be available on all property types so we should clearly document when and where this option can be used.