feat: implement anvil_{set,remove}BlockTimestampInterval API endpoints

[itegulov]

Two methods for controlling block timestamp interval (i.e. how much timestamp should increase in between of blocks):

#[rpc(name = "anvil_setBlockTimestampInterval")]
fn set_block_timestamp_interval(&self, seconds: u64) -> RpcResult<Metadata>;

#[rpc(name = "anvil_removeBlockTimestampInterval")]
fn remove_block_timestamp_interval(&self) -> RpcResult<NodeInfo>;