Missing timestamp(int) for timestamp from Unix Epoch

[TristonianJones]

The spec currently supports one-way conversion from timestamp to Unix epoch, but ideally it should be possible to convert from Unix epoch to timestamp as well.

See google/cel-go#357 for a dev report requesting the same feature. Round-tripping seems to be an expected use case.

• Add a new entry in the standard library function table docs.
• Add conformance tests to ensure such conversions are possible on all runtimes.