feat(core): add in-memory mempool for 4337 user operations

[rayyan224]

What changed? Why?
This PR adds an in-memory mempool implementation for ERC-4337 user operations in account-abstraction-core, so we can enqueue, prioritize, and dequeue operations deterministically.

Key Changes:
◦ Introduces Mempool trait:

▪ add_operation(...) (validates min gas requirement, inserts/replaces)
▪ get_top_operations(n) (returns up to n ops, filtered to lowest nonce per sender)
▪ remove_operation(hash) (removes from all indices)

◦ MempoolImpl maintains an implementation of this

[wlawt]

logic makes sense to me!

just to confirm my understanding of all the AA/ERC-4337 stuff going on ...

1. this in-memory mempool, is similar to the "off-chain mempool" that currently exists?
2. is the pub trait Mempool a shared trait that op-rbuilder uses to fetch the userOps to build bundles out of?
3. i'm assuming there'll be a follow-up PR to add the userOp to the in-memory mempool after it passes the validate_user_operation check in ingress-rpc?
4. i'm not sure if this makes sense, but, what's the role of the user_op_queue_publisher now that we'll have the mempool? is it there to act as a buffer?

[rayyan224]

logic makes sense to me!

just to confirm my understanding of all the AA/ERC-4337 stuff going on ...

1. this in-memory mempool, is similar to the "off-chain mempool" that currently exists?
2. is the pub trait Mempool a shared trait that op-rbuilder uses to fetch the userOps to build bundles out of?
3. i'm assuming there'll be a follow-up PR to add the userOp to the in-memory mempool after it passes the validate_user_operation check in ingress-rpc?
4. i'm not sure if this makes sense, but, what's the role of the user_op_queue_publisher now that we'll have the mempool? is it there to act as a buffer?

Nice questions, this mempool dosen't currently exist would actually be in the bundler i.e Rundler
2. Yep exactly
3. Yep the idea is that it will actually feed off the kafka events to keep the mempool in sync, this way we can keep it distributed, and single source of truth
4. It will still be needed as we need to tell the builder that there has been a change.

[DeStefaniAndrei]

lgtm