Add feat zeroize to cipher

[pronebird]

Seeing the following issue when trying to run cargo clippy on nym-vpn-client:

C:\Users\pronebird\Nym\nym-vpn-client\nym-vpn-core>cargo clippy --fix --lib -p nym-vpn-api-client --allow-dirty
    Checking nym-vpn-api-client v1.2.0-dev (C:\Users\pronebird\Nym\nym-vpn-client\nym-vpn-core\crates\nym-vpn-api-client)
error[E0432]: unresolved import `nym_crypto::ctr::cipher::zeroize`
  --> crates\nym-vpn-api-client\src\types\account.rs:5:30
   |
5  | use nym_crypto::ctr::cipher::zeroize::Zeroizing;
   |                              ^^^^^^^ could not find `zeroize` in `cipher`
   |
note: found an item that was configured out
  --> C:\Users\pronebird\.cargo\registry\src\index.crates.io-6f17d22bba15001f\cipher-0.4.4\src\lib.rs:35:9
   |
35 | pub use zeroize;
   |         ^^^^^^^
note: the item is gated behind the `zeroize` feature
  --> C:\Users\pronebird\.cargo\registry\src\index.crates.io-6f17d22bba15001f\cipher-0.4.4\src\lib.rs:33:7
   |
33 | #[cfg(feature = "zeroize")]
   |       ^^^^^^^^^^^^^^^^^^^

For more information about this error, try `rustc --explain E0432`.
error: could not compile `nym-vpn-api-client` (lib) due to 1 previous error

Looks like we're missing the feature flag for cipher which is re-exported via ctr.

This change is 

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[pronebird]

Closing in favour of refactoring the code in nym-vpn-client