Describe the enhancement

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

This is initially proposed by @LLFourn: bitcoindevkit/bdk#1178 (comment)

Use case

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

Implementation proposal

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