add lib crate-type to bdk-ffi to be able to reuse it for other bindings like bdk-dart

[kumulynja]

Description

This PR just adds "lib" as a crate-type besides "cdylib" and "staticlib". In this way bdk-ffi can be more easily wrapped and reused for other bindings like the dart bindings in bdk-dart. Without it, more code from bdk-ffi has to be duplicated to bdk-dart and the process becomes more complex and hacky.

Notes to the reviewers

I tested this PR in the following bdk-dart PR already: bitcoindevkit/bdk-dart#12
It would be good to get this merged here first to be able to remove the patch to my fork of bdk-ffi there.

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing
• I've added a changelog in the next release tracking issue (see example)
• I've linked the relevant upstream docs or specs above

[thunderbiscuit]

I'm not against this change per se, but I'd like to understand better why the plugin for dart needs it when others don't (JVM, Android, Swift, Python, React Native).

I took some time to clean this up a while back and my understanding was that the lib type is required in situations where your Rust code is used as a dependency itself, whereas for us here it's the compiled artifacts that are used by the plugins.

Again I'm not against it because it doesn't impact us negatively other than build times, but I want to make sure I know why each of those crate types are required (cdylib is for Android/JVM/Python, staticlib is for iOS).

[thunderbiscuit]

This is the only line that needs to be on this PR.