Disable access to pending txs from filters + subscriptions

[mdehoog]

Description
Currently op-stack chains can leak tx pool contents on shared nodes using two methods:

• HTTP call to eth_newPendingTransactionFilter and subsequent calls to eth_getFilterChanges
• WS eth_subscribe to newPendingTransactions

This PR disables these two methods by default (returning an error: pending tx filters are disabled). It also adds a new flag: --rollup.allowpendingtxfilters which optionally enables them.

Tests
Tested running geth locally. Without the option:

> curl -d '{"id":0,"jsonrpc":"2.0","method":"eth_newPendingTransactionFilter","params":[]}' -H "Content-Type: application/json" http://localhost:8545
{"jsonrpc":"2.0","id":0,"error":{"code":-32000,"message":"pending tx filters are disabled"}}

> wscat --connect ws://localhost:8546
> { "id": 1, "jsonrpc": "2.0", "method": "eth_subscribe", "params": ["newPendingTransactions"] }
< {"jsonrpc":"2.0","id":1,"error":{"code":-32000,"message":"pending tx filters are disabled"}}

With the --rollup.allowpendingtxfilters option:

> curl -d '{"id":0,"jsonrpc":"2.0","method":"eth_newPendingTransactionFilter","params":[]}' -H "Content-Type: application/json" http://localhost:8545
{"jsonrpc":"2.0","id":0,"result":"0x3fe165e46f7aa71c04f3e538839a029"}

> wscat --connect ws://localhost:8546
> { "id": 1, "jsonrpc": "2.0", "method": "eth_subscribe", "params": ["newPendingTransactions"] }
< {"jsonrpc":"2.0","id":1,"result":"0xf6f39af0ac3984a166ab78bc23a9aa9a"}

Additional context

https://status.base.org/incidents/m9fnx4p3bhp5

[trianglesphere]

I missed that tests need to be updated. Changes still look good though.

eth/filters/filter_system_test.go:266:10: assignment mismatch: 1 variable but api.NewPendingTransactionFilter returns 2 values
eth/filters/filter_system_test.go:323:10: assignment mismatch: 1 variable but api.NewPendingTransactionFilter returns 2 values
eth/filters/filter_system_test.go:918:10: assignment mismatch: 1 variable but api.NewPendingTransactionFilter returns 2 values (typecheck)

fixed