feat(optimism): Handle incoming flashblocks against current pending block number

[julio4]

Close #18191
Relevant #17858

When handling a received flashblock (in add_flash_block of Flashblock service):

• fetch latest block number to calculate pending block height
• Correctly handle flashblock based on its flashblock.metadata.block_number

[mattsse]

not something that is useful here imo

[mattsse]

how does this relate to #18183

[julio4]

how does this relate to #18183

It should implement this todo

reth/crates/optimism/flashblocks/src/service.rs

Lines 139 to 142 in 0ad8749

	} else {
	// TODO: store non sequential flashblock index for later processing
	debug!("ignoring early {flashblock:?} at current height {latest}");
	}

[mattsse]

thanks,
this is no longer relevant after we made some other simplifications