Data: Consider optimizing aggregate of _fields requests

[aduth]

Previously: #12930

In an assumed scenario where two data consumers each fetch an entity (or entity set) while providing _fields, it should be considered whether previous requests could be used to satisfy the requirements.

For example, the following should only require a single network request:

wp.data.select( 'core' ).getEntityRecords( 'postType', 'post', { _fields: [ 'title', 'content' ] } );
wp.data.select( 'core' ).getEntityRecords( 'postType', 'post', { _fields: [ 'title' ] } );

Related:

• https://core.trac.wordpress.org/ticket/43874