Batch tx broadcast RPC

### Please describe the feature you'd like to see added.

The intent of #31085 wasn't really *just* to be able to broadcast a "package" of size 1, but that it would be nice if the caller of `submitpackage` didn't need to sit down and figure out/make sure all their broadcasts match exactly some BIP-defined structure. Ideally I just give Bitcoin Core a pile of transactions and it figures it out - if there's two children and two parents it broadcasts them as two packages, etc. Maybe this is just something we get for "free" with cluster mempool and larger package relay (as long as the transactions are even related to each other, though it'd be kinda nice for Bitcoin Core to figure that out too), but any restrictions in `submitpackage` ultimately filter down through the ecosystem and imply restrictions on hundreds of internal interfaces (eg the LDK `BroadcasterInterface`) and APIs (eg Esplora/Electrum implementations will generally just call that method), so it'd be nice to be as generic as possible.

### Is your feature related to a problem, if so please describe it.

_No response_

### Describe the solution you'd like

_No response_

### Describe any alternatives you've considered

_No response_

### Please leave any additional context

_No response_

Start by reading issue #31085 and the existing submitpackage RPC behavior, since no implementation files or tests are named here. The proposed result would accept a pile of related transactions and determine suitable packages for broadcast without requiring callers to construct a BIP-defined structure.

Written by the indexing model from the issue text.