Add flexible format supports to DateTime::parse_from_rfc3339

[foxhlchen]

DateTime::parse_from_rfc3339 lacks of some iso-8601 format supports which makes it very hard to collaborate with tools such as GNU date.

This commit adds support for thoses formats generated by GNU data:

date --iso-8601=ns 2021-04-15T09:49:00,321433476+08:00

date --iso-8601=minutes 2021-04-15T09:49+08:00

date --iso-8601=hours 2021-04-15T09+08:00

Signed-off-by: foxhlchen [email protected]

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