Add some missing methods to the types in datetime

[jhendrixMSFT]

Added missing text (un)marshalers on some types so they're all uniform. Handle JSON nulls in JSON unmarshalers.
Cleaned up some missing punctuation in doc comments.

[github-actions]

API Change Check

APIView identified API level changes in this PR and created the following API reviews

sdk/azcore

[Copilot]

Pull request overview

This PR enhances the datetime package by adding missing text marshalers/unmarshalers to ensure all datetime types have consistent serialization support, and improves robustness by handling JSON null values in unmarshalers. It also cleans up documentation by adding missing punctuation to doc comments.

Key Changes

• Added MarshalText and UnmarshalText methods to Unix and PlainDate types for consistency with other datetime types
• Implemented JSON null handling across all UnmarshalJSON methods to prevent errors when deserializing null values
• Fixed missing periods in doc comments for String, MarshalText, and UnmarshalText methods across multiple types

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
sdk/azcore/runtime/datetime/unix.go	Added text marshaling methods, null handling, and refactored parsing logic into a shared helper function
sdk/azcore/runtime/datetime/unix_test.go	Added comprehensive tests for new text marshaling methods and null handling
sdk/azcore/runtime/datetime/plain_date.go	Added text marshaling methods, introduced jsonNull constant, and null handling in UnmarshalJSON
sdk/azcore/runtime/datetime/plain_date_test.go	Added tests for text marshaling methods and null/empty input handling
sdk/azcore/runtime/datetime/plain_time.go	Added null handling in UnmarshalJSON and fixed doc comment punctuation
sdk/azcore/runtime/datetime/plain_time_test.go	Added tests for null and empty input handling
sdk/azcore/runtime/datetime/RFC3339.go	Added null handling in UnmarshalJSON and fixed doc comment punctuation
sdk/azcore/runtime/datetime/RFC3339_test.go	Replaced generic test with focused null handling test
sdk/azcore/runtime/datetime/RFC1123.go	Added null handling in UnmarshalJSON and fixed doc comment punctuation
sdk/azcore/runtime/datetime/RFC1123_test.go	Added test for JSON null handling