Data: Consolidate onSubKey (and other helpers) to data module

[aduth]

A few higher-order reducer utilities have been implemented across Gutenberg to assist with managing @wordpress/data store reducer flows. One in particular has now been implemented identically across three modules: onSubKey:

• https://github.com/WordPress/gutenberg/blob/master/packages/core-data/src/utils/on-sub-key.js
• https://github.com/WordPress/gutenberg/blob/master/packages/notices/src/store/utils/on-sub-key.js
• https://github.com/WordPress/gutenberg/blob/master/packages/data/src/store/utils.js

Suggested at #9617 (comment) , perhaps we should consolidate this and other store-related reducer utilities to be an exported function of @wordpress/data. We already do this with other utilities such combineReducers.

[paaljoachim]

Hey @aduth Andrew.

Does this issue still have relevance?
As your away these days. I'll ping Greg @gziolo .