Create bitcoinj-based Electrum server

[msgilligan]

Some of the major tasks to implement this include:

1. Version upgrades of major dependencies (JDK 25, Jackson 3, JUnit 6, possibly Micronaut 5)
2. Type definitions for Electrum requests and responses (preferably in a separate module)
3. Electrum-compatible database module (likely using RocksDB)
4. Ability to quickly initialize a static database using Bitcoin Core chain state.
5. Ability to be a caching server relying on a trusted Bitcoin Core instance
6. Ability to sync directly with the Bitcoin network using bitcoinkernel-jdk
7. A daemon/application module tying it all together (probably using Micronaut, but suggestions are welcome)

[yuvicc]

Cool I'm interested! Will definitely work on this one.

For daemon/application (pt. 6) I think a single unified daemon will be a good imo with plugin system. Users can enable/disable features via configuration (e.g., enable Electrum server, enable caching, etc.). Also, this aligns with existing module cj-btc-walletd, which already uses this approach with injectable services.

I will create a separate branch for this one and then link it here for anyone who would like to review.

[msgilligan]

If you look at recently merged PRs you'll see that I've (finally) upgrade to Gradle 9 and the build now works on JDK 25.

I've also done some work on creating a Java interface for the Electrum JSON-RPC methods. I will post it as a WIP PR soon, if you are interested.

[yuvicc]

I've also done some work on creating a Java interface for the Electrum JSON-RPC methods. I will post it as a WIP PR soon, if you are interested.

Indeed, that will be helpful.

[yuvicc]

Opened draft rocksdb module for blockchain database.

#381