eth: add admin_clearTxpool api

[cuiweixie]

I think this rpc is necessary, at least for my scene, do need this rpc to clear tx pool.
reth already has this rpc method.
in paradigmxyz/reth#18539

[jwasinger]

Why do you need this?

The original reth-linked issue has the following rationale:

In some L2 architectures where the L1 blockchain acts as the sequencer, we need to maintain strict transaction ordering between L1 and L2 layers. This ensures proper mempool clearing and prevents ordering discrepancies that could arise from transaction persistence issues.

It's not clear to me how this functionality is facilitated by making the mempool clear an rpc method, or if Geth even cares about that use-case..

[cuiweixie]

In my case, this rpc can be used to clear tx pool when benchmark the chain using tools like https://github.com/0gfoundation/evmchainbench or https://github.com/cuiweixie/adventure/tree/leo/contract-deploy. Whose gas price is fixed generally, but basefee will go higher that fixed gas price, or txpool is full cause some tx got pending, those txs can not get on chain, when we stop the benchmark or restart the benchmark, before that, we need to clear tx pool.