feat: Polkadot connector

[penumbra23]

Please share the technical limitation of Caliper that you encountered.

Currently there is no support for Polkadot based chains (formerly known as Substrate chains). I guess expanding the support for different chains is something this tool wants.

Please detail your feature idea that could alleviate the limitation.

Using the PolkadotJS API as the main JS library for interacting with the Polkadot chain is pretty straightforward, the API is very direct and robust, mainly all apps use it when it comes to Substrate chains.

Please share some details about your use case if possible, and how the new feature would make Caliper a better performance benchmarking framework.

Adding support for Substrate chains not only increases the use case for the Polkadot chain, but any chain that's built using the polkadot-sdk, which include Kusama, all the Asset hubs, Astar, Moonbeam & Moonriver, Aleph Zero, ChainX and many others. To say in other words, killing a dozen flies with one slap.

Please share any suggestions about the new feature's code/configuration API (using formatted YAML segments or pseudo-code).

I've created a branch on my fork with the initial implementation and integrations tests.

I've added support for 2 possible scenarios:

1. signing happens with the signers configured in the network config
2. "offline signing" - any kind of signing is possible and the worker basically does the submission and awaiting of the confirmation

Here is the PR preview, any reviews, comments, suggestions are more than welcome!