Avoid unused_imports warning for std::convert::TryFrom.
Conditionally import it only when it is needed -
in sk_msg programs.
remove unused `std::convert::TryFrom` import to avoid the warning in the generated projects:
```
warning: unused import: `TryFrom`
--> myapp/src/main.rs:4:15
|
4 | convert::{TryFrom,TryInto},
| ^^^^^^^
|
= note: `#[warn(unused_imports)]` on by default
warning: `myapp` (bin "myapp") generated 1 warning
```
User may specify a program_type by prompt or CLI flag.
We then generate skeleton code to the ebpf program for a noop program of
that type.
Requires cargo-generate@main
Signed-off-by: Dave Tucker <dave@dtucker.co.uk>