feat: handling timedelta objects in the FriendlyEncoder class

[arthurbrenno]

I came to an use case in Blacksheep where I needed to return timedelta objects. This timedelta object was inside a Pydantic BaseModel. But I got the error TypeError: Object of type timedelta is not JSON serializable because essentials didn't handle timedelta objects. Leaving this small contrib. Hope it helps others too.

Changes:

• Added another if statement, that checkes if obj is an instance of a timedelta object.