Update OptimismPayloadBuilder field visibility

[0xKitsune]

This PR updates the visibility of OptimismPayloadBuilder fields to pub. This allows custom OP stack payload builders using the OptimismPayloadBuilder as an inner type to reference these variables.

pub struct OPStackBlockBuilder<EvmConfig> {
    inner: OptimismPayloadBuilder<EvmConfig>,
}

impl<EvmConfig> OPStackBlockBuilder<EvmConfig> {
    fn build_payload(&self) {
        // --snip--
        let config = &self.inner.evm_config;
    }
}

[mattsse]

this seems fine