Multiplex `eth_sendRawTransaction` to the builder.

[ferranbt]

Rollup-boost should multiplex the transactions sent with eth_sendRawTransaction to the builder too. Enable this behaviour with a flag.

[avalonche]

this is easier to do at the rpc endpoint level via nginx than natively inside rollup-boost due to restrictions in the jsonrpcsee library.

[ferranbt]

But with nginx you are adding an extra service to the mix, in this case the platform would do it natively.

Also, if rollup-boost had access to the transactions it could keep track on whether the builder is censoring them or not and act accordingly, for example, adding a few blocks from the proposer op-geth.