[blockchain] Add traits to reuse Blockchains across multiple wallets

[afilini]

Description

Add three new traits:

• StatelessBlockchain is used to tag Blockchains that don't have any wallet-specic state, i.e. they can be used as-is to sync multiple wallets.
• StatefulBlockchain is the opposite of StatelessBlockchain: it provides a method to "clone" a Blockchain with an updated internal state (a new wallet checksum and, optionally, a different number of blocks to skip from genesis). Potentially this also allows reusing the underlying connection on Blockchain types that support it.
• MultiBlockchain is a generalization of this concept: it's implemented automatically for every type that implements StatefulBlockchain and for every Arc<T> where T is a StatelessBlockchain. This allows a piece of code that deals with multiple sub-wallets to just get a &B: MultiBlockchain without having to deal with stateful and statless blockchains individually.

These new traits have been implemented for Electrum, Esplora and RPC (the first two being stateless and the latter stateful). It hasn't been implemented on the CBF blockchain, because I don't think it would work in its current form (it throws away old block filters, so it's hard to go back and rescan).

This is the first step for #549, as BIP47 needs to sync many different descriptors internally.

It's also very useful for bitcoindevkit/bdk_wallet#188.

Notes to the reviewers

This is still a draft because:

• I'm still wondering if these traits should "inherit" from Blockchain instead of the less-restrictive WalletSync + GetHeight which is the bare minimum to sync a wallet
• I need to write tests, at least for rpc which is stateful
• I need to add examples

Checklists

All Submissions:

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

New Features:

• I've added tests for the new feature
• I've added docs for the new feature
• I've updated CHANGELOG.md

[afilini]

I didn't like how StatefulBlockchain worked, so I just removed that trait since it was very similar to MultiBlockchain anyway. I've also renamed MultiBlockchain to BlockchainFactory (@notmandatory used this term in yesterday's call and I really liked it).

I've also updated all these traits to inherit from Blockchain rather than GetHeight + WalletSync: the rationale is that it's easier to relax these requirements later rather than tightening them as it would be an API break.

I've added tests and documentation, I'm feeling confident to remove the draft state from this PR.

[notmandatory]

Overall LGTM. I commented on some fixes needed in the rpc docs code, and a couple other suggestions.

[rajarshimaitra]

Concept ACK a817545

I think this would be really useful for multiple wallets to use the same backend.

Below are some nits and api questions I had..

[afilini]

Rebased and addresses most comments

[rajarshimaitra]

tACK 6f91598

Code changes looks good to me. Now I understand the intention better. This will be really useful..

Just one non blocking nit. Can be done in a following PR.

[notmandatory]

This one needs one more rebase to pickup the new MSRV CI changes and then it should be ready to go.

[rajarshimaitra]

tACK 8795da4