fix: temporarily require rand feature from libp2p-identity

[Rjected]

enr-cli was not building with the following error:

   Compiling libp2p-identity v0.2.8
error[E0433]: failed to resolve: use of undeclared crate or module `rand`
  --> /Users/dan/.cargo/registry/src/index.crates.io-6f17d22bba15001f/libp2p-identity-0.2.8/src/ecdsa.rs:98:43
   |
98 |         SecretKey(SigningKey::random(&mut rand::thread_rng()))
   |                                           ^^^^ use of undeclared crate or module `rand`

For more information about this error, try `rustc --explain E0433`.
error: could not compile `libp2p-identity` (lib) due to 1 previous error
warning: build failed, waiting for other jobs to finish...
error: failed to compile `enr-cli v0.5.1`, intermediate artifacts can be found at `/var/folders/65/m26_cf0d1lbdnp4dvzztr0th0000gn/T/cargo-installxhFN7G`.
To reuse those artifacts with a future compilation, set the environment variable `CARGO_TARGET_DIR` to that path.

So this will need the rand feature enabled until libp2p/rust-libp2p#5212 is merged