fix: set gas price to enforce priority fee to zero

[jcabrero]

This PR computes the gas price from the transaction is sent to the chain, and enforces it manually. This ensures that neither the GasFiller, nor the chain set a default priority fee or gas cost.

Other small changes:

• Added a Phala example attestation to the repository.
• Changed, the nilCC simulator to also send Phala attestations.
• Added a log entry to check the effective cost of each transaction invocation (we can consider silencing it if it is too noisy).