tick: refine the MIN, MAX values for timestamps in fmt module

[martintmk]

The tick crate exposes the following formats for SystemTime:

• Iso8601
• Rfc2822
• UnixSeconds

All formats above expose MIN and MAX values. Currently, the MIN points to UNIX_EPOCH while the underlying jiff crate that we are using for parsing and serialization allows even lower min values.

Make the following changes across all 3 formats:

• Update the MIN field value to refer to a MIN value as of 1 January 0001 00:00:00 UTC. Note: while jiff supports even lower min values, the Rfc2822 format cannot display these. So it's better that we just align to 1 January 0001 00:00:00 UTC as it is supported by all 3 formats and for the purpose of system time handling it should be sufficient.
• Introduce UNIX_EPOCH field that that refers to UNICH epoch