$merge/$patch pre-processing keywords for schema extension

[epoberezkin]

Proposed by @fge

The reason to implement them is to allow to extend existing schemas (e.g. meta-schemas, but equally important for any large schema).

For non-recursive schemas allOf/anyOf keywords can be used, but for recursive schemas there is no extension mechanism.

https://github.com/daveclayton/json-schema-validator/wiki/v5:-merge
https://groups.google.com/forum/#!topic/json-schema/sztyQoNlnDM

@fge, syntax you propose is a bit ambiguous, it would be good to clarify and add to v5 proposals page.

[yyscamper]

+1, I am also looking forward to this feature. In my case, the additionalProperties doesn't work as my expectation when I use the allOf to merge two-schemas, but merge can.

[epoberezkin]

For small/non-recursive schemas you own you can always refactor at least...

[epoberezkin]

See $merge and $patch keywords.