Extend implementation of to_ixdtf_string to more types

[nekevss]

This PR continues the work on implementing toString functionality in temporal_rs.

It completes the following:

• Adds to_ixdtf_string functions on DateTime, Time, ZonedDateTime, and Instant.
• Adds ToStringRoundingOptions options type.
• Adds an IxdtfStringBuilder struct to assist with building ixdtf strings.

So far I have verfied that the PR has a 100% conformance rate for PlainDateTime. I plan to test the others, but any poentially issues that may arise with those could always be completed in subsequent PRs.

[jasonwilliams]

LGTM!