Lining Up The Big 3.0 PRs

This issue aims to put in writing some of the discussions and brainstorms we had in person last week. It's an attempt at laying out the big PRs that we see as potentials for the 3.0 release, and how they relate to each other. Namely, some of them are simply lower-hanging fruits that can be quicker wins, and others have dependencies between them, and so should be approached in order. Feel free to add to this, comment, etc. Note that this list does not imply that all of those features will make it into the 3.0 release. They are simply the potential ones. Where exactly the line ends up being drawn is still in the works.

The 9 features discussed were the following, in no particular order:
- [x] 1. Locking UTXOs (#259)
- [x] 2. Wallet events (#319)
- [ ] 3. TxBuilder becomes create_psbt/PsbtParams (#297)
- [ ] 4. Wallet doesn't handle signing (#235)
- [ ] 5. Multi-Keychain support (#318)
- [ ] 6. Intent tracker/Broadcast queue (#257)
- [ ] 7. Single Sync flow ([issue](https://github.com/bitcoindevkit/bdk/issues/2057))
- [ ] 8. Bump bdk_chain (miniscript 13 + other goodies)
- [ ] 9. Wallet birthday #368

Of those, we identified UTXO locking as a potential ready-to-go, with wallet events as close second (maybe needs a bit more work now with the new block events). Then these unlock (3) and (8) -> (4). The rest are more "floaters" and can arrive in any order. Numbers (5), (7), and (9) are probably the ones that are less ready at the moment and will need more review/design before being fully ready.

Here is the diagram representing basically the above:

```txt
(1)
|
|
| Floaters
(2) ----------
/ \ 5, 6, 7, 9
/ \
/ \
(3) (8)
|
|
|
(4)
```