Added serde feature to sp-consensus-aura (#14166)

This is followup of #13027.

`Aura` need to enable `serde` feature in dependent crates, otherwise
test-substrate-runtime compilation fails with the following error if
`serde` is enabled:

```
  error: cannot find macro `format` in this scope
    -->
/home/miszka/parity/10-genesis-config/substrate-master/primitives/consensus/aura/src/lib.rs:50:3
     |
  50 |         app_crypto!(ed25519, AURA);
     |         ^^^^^^^^^^^^^^^^^^^^^^^^^^
     |
     = help: consider importing one of these items:
             scale_info::prelude::format
             sp_application_crypto::format
     = note: this error originates in the macro
`$crate::app_crypto_public_common_if_serde` which comes from the
expansion of the macro `app_crypto` (in Nightly builds, run with -Z
macro-backtrace for more info)
```