re-export uniffi

[vivianjeng]

Problem

Now the test e2e needs to import both mopro-ffi and uniffi to make it generate bindings

Details

We can simplify this by re-exporting uniffi in mopro-ffi
so the users can just use

[dependencies]
mopro-ffi = "0.1"

[build-dependencies]
mopro-ffi = "0.1"

instead of

[dependencies]
mopro-ffi = "0.1"
uniffi = "0.29"

[build-dependencies]
mopro-ffi = "0.1"
uniffi = { version = "0.29", features = ["build"]}

We can also make it optional
e.g.
user can choose to use the 2nd way if he wants to use specific version of uniffi
and use a feature flag uniffi to export the default uniffi version
or it becomes

[dependencies]
mopro-ffi = {version = "0.1", features = ["uniffi"]}

[build-dependencies]
mopro-ffi = {version = "0.1", features = ["uniffi"]}

Acceptance criteria

If the user removes uniffi dependencies, mopro-ffi can still generate bindings