Add 'expired' method to Signed class

[sechkova]

Description of issue or feature request:
Add a method of Signed metadata class returning information about metadata expiration given a reference time.

Current behavior:
Signed class already contains expires, a metadata expiration datetime object.

Expected behavior:
A method exists that checks the 'expires' attribute against a reference time.
def is_expired(self, reference_time: float) -> bool

[sechkova]

tuf.formats already provides some convertion functions:
https://github.com/theupdateframework/tuf/blob/2c360d0538f73a59854add867234f48a47f7d9a3/tuf/formats.py#L616
https://github.com/theupdateframework/tuf/blob/2c360d0538f73a59854add867234f48a47f7d9a3/tuf/formats.py#L646

[joshuagl]

Is a UNIX timestamp the preferred type for reference_time? Rather than a raw expiration string or a Python datetime object?

[lukpueh]

What's the intended use case? Would any of the datatypes be more convenient for the caller, that is which datatype would require the least preliminary conversion?

I lean towards making reference_time a datetime object, for consistency with the type of the Metadata expires field. And I'd also make reference_time optional and default to datetime.now().

[joshuagl]

Fully agreed with both comments. I have a WIP patch that takes a datetime object for reference_time and defaults to datetime.now() when no reference_time is passed.

I believe the motivating use-case is expiration checks in the experimental client, so I'll wait for @sechkova to comment on whether a datetime object is useful.

[sechkova]

No strong preferences, the client will use whatever we choose :)
I agree with using datetime objects plus the datetime module seems to be less platform dependent than time (which can be used to return UNIX time).

Let's make sure we enforce UTC times everywhere.