Update MMR Runtime API with functionality to generate MMR proof for a series...

Update MMR Runtime API with functionality to generate MMR proof for a series of leaf indices  (#10635)

* updated mmr rpc api with functions for batch generation of proof

* update code comments

* fix build errors

* added tests to mmr-rpc

* add tests to pallet-mmr

* update comments

* minor comment fix

* remove unused variables

* fix rust doc errors

* refactor mmr runtime api

* fix tests

* minor fix

* minor fix

* fix node-runtime

* revert to initial api

* impl from proof fot batchproof

* minor fix

* minor fix

* use explicit functions to convert btw batch proof and single proof

* minor fix

* add new variant to mmr error

* fmt

* update conversion to single leaf proof

* fix style nit

Co-authored-by: Adrian Catangiu <adrian@parity.io>