ethrpc: strictness validation level when unmarshalling blocks and txns

[pkieltyka]

This PR introduces StrictnessLevel on a ethrpc.Provider which effects how the provider will validate block and transaction data from nodes at the time of unmarshalling the json into a runtime type. Options >= 1 will perform rpc client-side checksum validation of the data coming from the nodes. Option 0, disables client-side validation.

The strictness levels:

• 0: disabled, no validation on blocks or transactions (default)
• 1: semi-strict transactions – validates only transaction V, R, S values
• 2: strict block and transactions – validates block hash, sender address, and transaction signatures

Usage

// default, no validation – we trust values from the node
provider, err := ethrpc.NewProvider(nodeURL)

// semi validation
provider, err := ethrpc.NewProvider(nodeURL, ethrpc.WithSemiValidation())

// strict validation
provider, err := ethrpc.NewProvider(nodeURL, ethrpc.WithStrictValidation())

// we also have ethrpc.WithStrictness(x) where you can pass the specific value, the above is sugar