Allow sub-millisecond resolution in `LocalTime`

[sgoll]

Description

Looking at the code of LocalTime, it seems that it explicitly allows only milliseconds as sub-second resolution. It is unclear (at least, to me) why this is the case. It seems unintuitive because both LocalDateTime as well as DateTime allow arbitrary precision for the sub-second part.¹

I would like to suggest lifting this limitation in LocalTime to bring it closer to LocalDateTime and DateTime.

This has come up in graphql-rust/juniper#1270 (comment) where serialization of the most common Rust time libraries is restricted to millisecond resolution despite all of them supporting nanosecond resolution internally.

Unfortunately, this could be considered a breaking change because it places the burden of parsing sub-millisecond resolution on downstream consumers.²

Footnotes

1. Despite neither the documentation of LocalDateTime nor DateTime indicating so. ↩

2. Although I would argue that most parsers should be able to handle the introduction of additional digits without any significant changes. ↩