decode panics on wasm32-unknown-unknown when validate_exp is true

[BrandonDyer64]

fn get_current_timestamp() -> u64 uses SystemTime::now() to get the current unix timestamp. Unfortunately, SystemTime functions panic on wasm32-unknown-unknown.

There are a couple of approaches to fix this:

1. Switch to chrono for getting the timestamp

   pub fn get_current_timestamp() -> u64 {
       chrono::Utc::now().timestamp() as u64
   }

2. In Validation, allow the user to supply a timestamp

   /// Supply a custom timestamp for checking `exp` and `nbf` claims.
   /// If `None`, will use `SystemTime::now()`.
   ///
   /// Defaults to `None`.
   pub current_timestamp: Option<u64>,

As a long-term solution, I prefer option 2 as it adds no cost, doesn't inflate the dependency tree, and allows for better customization.

[Keats]

3. Do like Chrono does: https://docs.rs/chrono/latest/src/chrono/offset/utc.rs.html#84-108 and have a different function if it's in WASM just importing the js sys lib in wasm

2 is not great because that means you now need to set the timestamp everytime you use the library while I'm expecting a lot of Validation struct to be mostly static

[BrandonDyer64]

@Keats There is an open PR that adds specific wasm support (#318), but it needs to be merged

[Keats]

Yes. As mentioned in the PR, it's missing CI

[BrandonDyer64]

@Keats The structure of Validation does not generally imply being static as the sub field is user specific. The original PR I submitted allowed for a user to choose to add a custom exp timestamp if they want.

I made another PR that uses js_sys when on wasm32-unknown-unknown, but I heavily urge you to please find a way to let the user specify a timestamp for validation