Add stats + CI improvement

[x3c41a]

Parametrise all the examples tests with network_address and private key (eventually through .env but can be a parameter at first).
Then propagate the changes all the way to the CI, through just and intermediary functions.

Let's focus on store_big_data.js - we need some stats for storing data.

TODO

• check TODOs, basically, I would like to connect local docker Kubo to the live testnet RPC node, to let the test pass: https://github.com/paritytech/polkadot-bulletin-chain/pull/182/files#diff-6d65d6c2df0c42e7b9fed55b65594037319f9f9211f07dc748b83f1bf738a8a6R561-R562
• Add simple statistics to the output (let's better tune this with local run and then try with westend):

File size | Transaction size (CHUNK_SIZE) | Average block utilization | Number of blocks  | Time elapsed |
----------------------------------------------------------------------------------
32 MIB | 1 MiB  | number of `store` transactions per block | ? | ? | ? |