rust: Fix daylight saving inaccuracy

[bact]

The timezone offset should use the offset of the date in the parsing (input data), not the current date (today). Or it risks to get a wrong offset due to shift from daylight saving time (DST).

Example: Decoding "2024-03-11T01:02:03" (no timezone, before DST started) while running the code today in July (DST active, UTC+1) previously produced 2024-03-11T01:02:03+01:00 which is wrong, since March 11 was actually UTC+0. It now correctly produces 2024-03-11T01:02:03Z.

Note that this issue will not occur with a valid SPDX 3 document since SPDX 3 DateTime requires it "always expressed in UTC time zone" so no DST and no issue. But as shacl2code can be generic, the issue may occurs with non-SPDX use cases.

[bact]

@stondo please kindly review at your convenience

[github-actions]

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[JPEWdev]

Is ti possible to add a test for this?

[bact]

Will add tests, thanks

[bact]

Test added.

The test set locale to New York time.

Then create two date times, one noon in summer and one noon in winter, using the generated decode_date_time().

Then check if each of them get a correct time zone component in ISO date output from the generated encode_date_time() (should get "-04:00" for summer, "-05:00" for winter).

chrono Rust dependency is set to 0.4.31 because we use fixed_offset().