eth, cmd: add RollupNetrestrictTxPoolGossipFlag and RollupTxPoolTrustedPeersOnlyFlag

[nonsense]

This PR is:

1 . Adding a rollup.netrestricttxpoolgossip string flag, so that the node gossips and accepts gossips of transactions only to a restricted set, if one is specified.
2. Adding a rollup.txpooltrustedpeersonly bool flag, so that the node gossips and accepts gossips of transactions only to and from its trusted peers.

If none of the flags is specified, no peers are filtered, i.e. gossiping works for all peers.

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• Introduces the RollupNetrestrictTxPoolGossipFlag flag
• Introduces the RollupTxPoolTrustedPeersOnlyFlag flag
• Modifies TxPool(peer) in Backend to return an actual transaction pool or nil transaction pool, depending if filtering is enabled with a given flag.
• TxPool(peer) also returns if the peer is allowed for gossiping (used for short-circuiting incoming messages, so that they get dropped)

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Related: ethereum-optimism/optimism#17751

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Fixes: #717

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op-devstack test: ethereum-optimism/optimism#18072

[pcw109550]

We may update the forkdiff(https://github.com/ethereum-optimism/op-geth/blob/optimism/fork.yaml) to track the geth diff.

[pcw109550]

This only takes care of outbound connections. To truly restrict tx gossiping, we must also prohibit inbound connections from disallowed peers. In other words, external peers' transactions must not be gossiped in to the node.

We may need to explore other control flows that may create outbound connections for tx gossiping.

[pcw109550]

Worth to mention that the new flag will be the same information restriction level with GETH_ROLLUP_DISABLETXPOOLGOSSIP, which restricts inbound AND outbound (for the gossip)

[pcw109550]

I have written the PoC code based on this, at #713, using txpool to not manually short circuit tx gossiping.

[nonsense]

Thanks @pcw109550 , returning a Nil or real transaction pool per peer is a nicer and more succinct solution!

[nonsense]

Change-set here is pretty trivial, but we nevertheless validated it with a test at ethereum-optimism/optimism#18072 . Adding an acceptance test on op-geth side is a bit more elaborate, and I am not sure if it is worth the effort to increase the diff for this functionality. I see we didn't add one for NilPool.

I added a unit test for TxGossipNetRestrict.

[sebastianst]

And as discussed in Slack, we also want to restrict receiving & accepting incoming tx gossip. @nonsense