fix(datetime): account for allowed values when setting default date

[liamdebeasi]

Pull request checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)
  • Some docs updates need to be made in the ionic-docs repo, in a separate PR. See the contributing guide for details.
• Build (npm run build) was run locally and any changes were pushed
• Lint (npm run lint) has passed locally and any fixes were made for failures

Pull request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe):

What is the current behavior?

Issue URL: resolves #24722

What is the new behavior?

• Creates separate DatetimeParts interfaces for today's date (which is now immutable) and the "default" date which. The "default" date is used as a fallback when there is no value set but we need to render some kind of date in the UI.
• Updates the implementation of datetime to either use the default date or today's date. Anything that explicitly calls out today's date now uses todayParts (for example, getCalendarDayState, which highlights today's calendar day regardless of the *Values props or max/min).
• Renames getDefaultPart to getActivePartsWithFallback so as to not collide with defaultParts.
• Adjusts the "default" date so that it is use values specified by *Values props (if provided)
• Updates the workingParts to default to default to defaultParts (this is done by adjusting the fallback in processValue)

Does this introduce a breaking change?

• Yes
• No

Other information

[liamdebeasi]

This is blocked by #26121 and #26120