feat: support fraction part in timestamp

[discord9]

I hereby agree to the terms of the GreptimeDB CLA

What's changed and what's your intention?

allow parsing Timestampe wiht fraction part like 1970-01-01 00:00:00.042, which is supported in arrow's Array cast, so it make sense to maintance consistency

• Summarize your change (mandatory)
  support decimal fraction part in DateTime::from_str , i.e. 1970-01-01 00:00:00.042

Checklist

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.
• This PR does not require documentation updates.

Refer to a related PR or issue link (optional)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (e375060) 85.66% compared to head (424d4fa) 85.66%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3272   +/-   ##
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  Coverage   85.66%   85.66%           
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  Files         859      859           
  Lines      140780   140796   +16     
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+ Hits       120595   120609   +14     
- Misses      20185    20187    +2     

[waynexia]

LGTM

[fengjiachun]

LGTM