Add support for testnet4

[notmandatory]

There is a core PR (bitcoin/bitcoin#29775) in the works to update testnet3 to testnet4. If/when this is merged we should update the BDK testnet genesis hash to use the new one. Also examples and docs should be updated, especially changing testnet3 electrum/esplora URLs to equivalent ones for testnet4.

[BinChengZhao]

Dear friends,

Recently, I have implemented support for Testnet4 in both rust-bitcoin and Blockstream/electrs. I am also keenly interested in contributing to this feature in BDK.

I noticed that the current plan is to replace Testnet3 with Testnet4 in BDK once the relevant changes are merged into the Bitcoin Core repository. However, I am concerned that this direct replacement might pose compatibility challenges for users, requiring them to choose between the two test networks in a given version.

In the rust-bitcoin community, we decided to add a TestnetVersion to accommodate both existing test networks as well as any future ones.

I would love to hear the BDK community's thoughts on this and am eager to contribute to implementing Testnet4 support in BDK based on our collective consensus.

[notmandatory]

@BinChengZhao it looks like the proposed rust-bitcoin/rust-bitcoin#2945 is a breaking change and we're not planning to change our rust-bitcoin minor version for the bdk 1.0 milestone. But even with current bdk 1.0-alpha you are able to use a custom genesis hash to support any testnet you want.

For now I'm targeting this issue, with full enum support for different testnet versions, to a future bdk 2.0 milestone.

[BinChengZhao]

@BinChengZhao it looks like the proposed rust-bitcoin/rust-bitcoin#2945 is a breaking change and we're not planning to change our rust-bitcoin minor version for the bdk 1.0 milestone. But even with current bdk 1.0-alpha you are able to use a custom genesis hash to support any testnet you want.

For now I'm targeting this issue, with full enum support for different testnet versions, to a future bdk 2.0 milestone.

Thank you very much for your hint, I will try it out by custom genesis hash first. :)