Add generic to LocalChain-based types

[evanlinjin]

Description

This is so that we can do future upgrades to LocalChain (and associated types) in a backwards-compatible way.

@LLFourn mentioned that bdk_chain changesets can be non-changing. A LocalChain with monotone changesets can be a different type completely (for example).

However, adding "metadata" to localchain is important for merkle proofs (for electrum, cbf, etc) as we need to store the header somewhere - and it makes sense for it to be in LocalChain.

Notes to the reviewers

I will add more info later

Changelog notice

• Added a generic B to LocalChain types.

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

[evanlinjin]

I just realized that since local_chain::ChangeSet<BlockId> is the same as local_chain::ChangeSet, we actually do not need this PR. We can do this change later and still be backwards compatible.