Add `populate_anchor_cache` method to `BdkElectrumClient`

[evanlinjin]

Can be done in a follow up, but we can have a populate_anchor_cache method. This will speed things up even more for subsequent syncs.

Originally posted by @evanlinjin in #1957 (comment)

To Do (bdk)

• Add BdkElectrumClient::populate_anchor_cache
• (Maybe) Convenient method on TxGraph
• Update examples

To Do (bdk_wallet)

• Convenient method on Wallet
• Update examples

[Her-Code]

Hey, I'd love to work on this

[evanlinjin]

@Her-Code thank you for offering. I've assigned you

[Her-Code]

@evanlinjin Awesome, thanks! I’m on it

[kwsantiago]

Hi @Her-Code are you still working on this?

If not, I'd like to give this a go if possible.

[Her-Code]

Heyy @kwsantiago , yes I'm still working on it.