Make the timestamp system in Activity closer to that of the Discord SDK, and more user friendly

[ewpratten]

Checklist

• [X] I have read the Contributor Guide
• [X] I have read and agree to the Code of Conduct
• [X] I have added a description of my changes and why I'd like them included in the section below

Description of Changes

After working with this crate in a recent project, the largest headache I encountered was the lack of ability to set just a start timestamp on activity (to get the (xx:xx elapsed) message in my status).

This PR adds two new methods to ActivityBuilder and slightly changes IntoTimestamp and Timestamps to add this ability.

I have added ActivityBuilder::start_timestamp and ActivityBuilder::end_timestamp which both set only their respective timestamp values when called. I have also deprecated ActivityBuilder::timestamps (although this can be undone), and changed its code to just call start_timestamp and end_timestamp. (Important note, calling both methods together will set a time range, like timestamps does)

Both the start_timestamp and end_timestamp now take a generic T: IntoTimestamp instead of an impl IntoTimestamp since the Rust compiler seems to dislike the old way, throwing errors when the method is called.

I have added an IntoTimestamp impl for i64, so crate users can easily enter a unix timestamp as an integer (more useful for testing than anything).

Finally, to make setting only one timestamp at a time possible, I have added serde annotations to Timestamps to skip serialization of empty fields.

I'll be keeping an eye on this PR for any requested changes, or comments.

Related Issues

None

[Jake-Shadle]

I don't think there is any need to deprecate timestamps, there are situations where where having a single method can be more convenient compared to 2 separate builder methods. Everything else looks good though (well, other than the rustfmt issues)!

[ewpratten]

Alright. Both of those should be addressed now.