feature: add /create_validator endpoint on exposer

[Anmol1696]

Overview

Currently validators are set with bash scripts with PostStartHook on the validator node. If we have to set the validator from external setup, then we need some admin functions on the exposer.

Proposal

Allow ability to run exposer binary ontop of chain image using chain binary for performing os.exec commands.

Future work

Exposer will be going to have multiple admin functions down the line, this will be the start of the admin functions on the exposer.

Related work

This feature is required for #265 where the validators would have to be initailized externally from the validator nodes itself.

Also we can update the current PostStartHook to be simpler, with making a call on the exposer, instead of the validator node itself.