Put building of ockam_app create behind a feature flag

[mrinalwadhwa]

Building ockam_app requires specific OS packages
https://github.com/build-trust/ockam/blob/develop/tools/nix/parts/rust.nix#L46-L61 and typescript code.

This hurts the experience of new developers trying to build ockam rust code with cargo build

https://github.com/build-trust/ockam/tree/develop/implementations/rust/ockam/ockam_app
https://github.com/build-trust/ockam/blob/develop/Cargo.toml

cargo build in the root of ockam repo should succeed if a recent version of Rust is installed

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[mrinalwadhwa]

@davide-baldo This came up in one of our conversations earlier this week. It came up again in a conversion with @adiSuper94 on discord as well.

[adiSuper94]

For me on ubuntu running, sudo apt install libwebkit2gtk-4.1-dev did the trick.

[adiSuper94]

@mrinalwadhwa I am not sure if the fix will be a trivial change to the root cargo toml.
rust-lang/cargo#4942 (comment)

PLMK if I am missing some thing.

[mrinalwadhwa]

@adiSuper94 you're right, hmm.