feature: add `/create_validator` endpoint on exposer

## 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.