Include gas_price in block schema

[antazoey]

Please check if the PR fulfills these requirements

• The formatter, linter, and tests all run without an error
• Tests for the changes have been added (for bug fixes/features)
• Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

Includes property from response gas_price in block schema

• What is the current behavior? (You can also link to an open issue here)

gas_price is missing from block schema

What is the new behavior (if this is a feature change)?

gas_price is available in the block

Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)

Other information

[war-in]

Thanks for contributing! Tests are failing because RPC doesn't support gas_price in the block (even in the newest specification). The solution could be to create a new dataclass "GatewayBlock" (in the client_models.py) which extends StarknetBlock and adds gas_price field. We don't want to modify the StarknetBlock dataclass, because it's compatible with RPC specification.