Accept all RFC3339-compliant timestamps

[graebm]

Issue: aws_date_time did not accept "2024-02-23 23:06:27+00:00", despite that being a valid RFC 3339 timestamp.

Background:

• ISO 8601: is a standard for date and time, but has a lot of options in it.
  • aws_date_time has enums for ISO_8601, but does not claim to support every single option
  • One option it did support though, was Basic vs Extended format.
    • Basic: no separators between "YYYYMMDD" and "HHMMSS"
    • Extended: has separators between "YYYY-MM-DD" and "HH:MM:SS"
• RFC 3339: details a subset of ISO 8601 for use in internet timestamps
  • Smithy 2.0 says that timestamps should use RFC 3339
  • aws_date_time does not fully support RFC 3339. The unsupported features in the string above were: 1) Space instead of "T" between date and time 2) "+00:00" offset instead of "Z" after time
• I personally found the parsing code for ISO_8601 hard to follow, and the ISO_8601_BASIC code was a big copy/paste of that.

Description of changes:

• The code is rewritten to be simpler. It is no longer a state machine.
• Combine parsing code for ISO_8601 and ISO_8601_BASIC into 1 function.
  • The parse is lenient now, accepting Basic or Extended timestamps, regardless of whether the ISO_8601 or ISO_8601_BASIC enum is passed in.
    • Our ISO_8601 code already allowed separators to be omitted from time, which shows evidence that lenience is good.
    • This matches the leniency of Python's dateutil.parser.isoparse(str) which allows "2024-02-23T23:06:27Z" or "20240223T230627Z" or "2024-02-23T230627Z" or "20240223T23:06:27Z"
  • Allow space instead of "T"
  • Allows lowercase "t" and "z"
  • Support offsets like "+12:34", instead of just "Z"
  • Allow ",123" for fractional seconds, not just ".123"
    • The Basic parse code had a bug where did not expect any characters between seconds and fractional-seconds. This is wrong. Python's dateutil rejects this.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 96.96970% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 83.04%. Comparing base (15a2534) to head (f5b90ec).

Files	Patch %	Lines
source/date_time.c	96.96%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1093 +/- ## ========================================== - Coverage 83.15% 83.04% -0.12% ========================================== Files 56 56 Lines 5813 5762 -51 ========================================== - Hits 4834 4785 -49 + Misses 979 977 -2 ```

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