retry FCU if it returns SYNCING

[akundaz]

We want to retry this endpoint to not miss block production. If it returns "syncing" we should retry until we're ready.

Context: When the sequencer sends fcu with attributes, it signals to the builder to start building the next block. If builder answers with the response "SYNCING", we infer that it's not healthy, i.e. it doesn't have the parent block ready to build the next block. So we want to give the builder a bit more time to get that parent block.

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[SozinM]

this also should be configured and invoked via a flag
i think it's still useful to have in case of outages to alleviate them

[SozinM]

Please also add metric (counter) with the number of retries that we performed

[avalonche]

can use a library for this? and generalised function so it can be used across different methods

[akundaz]

can use a library for this? and generalised function so it can be used across different methods

Sure, there are a lot of options for this. I'll go with https://github.com/Xuanwo/backon, since it looks like its also used by reth (though not much), unless people have stronger opinions

Actually I'll do this in a separate PR because we want this out soon