[MaterialDatePicker] [MaterialTimePicker] Provide a widget that allows the user to select a calendar date, and optionally a time

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**Is your feature request related to a problem? Please describe.**
I haven't been able to find a way to add a view to the `MaterialDatePicker` that will allow users to switch to time and vice versa. Not all user cases are the same and the current `MaterialDatePicker` and `MaterialTimePicker` are not helping me to achieve my user requirements. My app might have multiple date/time fields. These are represented by `text field` in a display state (not focusable - users can not type a date and/or time). The purpose is to minimize data entry errors and maximizes efficiency.

**Describe the solution you'd like**
1. An inline date/time picker will be best. I know this not something that will happen overnight but this will be the preferable experience
OR
2. Expose the `HeaderToggle` accessory. This by default can be the switch from the calendar to input calendar or it can be styled and used to switch from the datepicker to the timepicker.

This screen below shows what I need to achieve from the design point of view.

Design Screen

**Describe alternatives you've considered**
Expose the neutral button in the fragment that hosts the picker. This will allow to switch from date to time and vice versa

**Additional context**
Provide a widget that allows the user to select a calendar date, and optionally a time.

iOS Screen

No files, tests, or entry points are named. Start by reviewing the existing MaterialDatePicker and MaterialTimePicker APIs, including the HeaderToggle and neutral-button alternatives described here. Done would require an agreed design and a widget that supports calendar-date selection with optional time selection and switching between them.

Written by the indexing model from the issue text.