Wallet's persistence should have a fixed "header" section

**Describe the enhancement**

Add a fixed header section to the wallet's persistence.

This is initially proposed by @LLFourn: https://github.com/bitcoindevkit/bdk/pull/1178#discussion_r1379454497

**Use case**

This can be used to store data that are mostly unchanging such as network type, version, etc.

**Implementation proposal**

```rust
pub struct Database: PersistBackend {
fn write_header(&mut self, header: H) -> Result<(), Self::WriteError>;
fn load_header(&self) -> Result;
}
```