feat: add testnet4 network

[reez]

Description

Resolves #663

Testnets are tough right now, testnet4 is another option we should give people to help them build with bdk-ffi.

@ValuedMammal added this same thing it in bdk, which was included in the bdk_wallet 1.1 release

We should have it in bdk-ffi as well.

Currently we rely on bitcoin-ffi for our Network type so I think our only option if we use bitcoin-ffi is to update bitcoin-ffi and then pull that new version of bitcoin-ffi with testnet4 into bdk-ffi.

Notes to the reviewers

Points to a PR branch in bitcoin-ffi.

Changelog notice

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

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR

[reez]

Both bdk-swift and bdk-jvm build locally, 3 CI failures look more like a possible uniffi problem.

Importing bdk-swift into an Xcode project gives me same errors as seen on CI, name clash/redeclaration problems.

@thunderbiscuit have you seen this same thing on bdk-jvm in your Issue619/bitcoin-ffi related explorations?