Implement TypeInfo for Duration

[nazar-pc]

Something in Substrate was merged recently, probably paritytech/substrate#13302, which made our code no longer compile with this error:

error[E0277]: the trait bound `std::time::Duration: scale_info::TypeInfo` is not satisfied
   --> crates/sp-consensus-subspace/src/lib.rs:479:1
    |
479 | / sp_api::decl_runtime_apis! {
480 | |     /// API necessary for block authorship with Subspace.
481 | |     pub trait SubspaceApi<RewardAddress: Encode + Decode> {
482 | |         /// The slot duration in milliseconds for Subspace.
...   |
534 | |     }
535 | | }
    | |_^ the trait `scale_info::TypeInfo` is not implemented for `std::time::Duration`

Encoding and decoding is supported for Duration, so TypeInfo should be too.