Improve dateTime* type support

[ivan4th]

Use time.Time to represent dateTime* types (breaking change, but I think it makes sense)
Support dateTimeMicroseconds and dateTimeNanoseconds

Helpful pointers (besides RFC 7011):
Timesamp handling in the Wireshark dissector for IPFIX
NTP Timestamp format

[vmwclabot]

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[dreamtalen]

Thank your contribution @ivan4th!
Since this is a performance-critical component and we're using it in another open source project at https://github.com/antrea-io/antrea/tree/main/pkg/agent/flowexporter, we may need more time to evaluate the performance impact from changing uint32 to DateTime type.
May I ask your purpose for this PR: just change the type to meet the RFC 7011 standard (actually dateTimeSeconds data type is an unsigned 32-bit integer according to https://www.rfc-editor.org/rfc/rfc7011.html#section-6.1.7)? Or you need DateTimeMicroseconds, DateTimeNanoseconds in your project?
Also, there are multiple test failures in the Github CI, could you please take a look at them? Please let us know if you need any help on refactoring the CI tests.
Thanks!

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