Improved validate unit to handle strings and to raise exception for invalid units

[cfulton]

Improved the validate_unit method so that it can handle both string and enums.

• Created set of all supported units. Set includes current enums and consts in units.py
• Handled if unit was string or enum.
• Checked if provided unit is in set of all supported units. Return unit if it is in set.
• raise new InvalidUnitOfMeasureException if we didn't return a valid unit.
• Changed return to always be string and updated implementation.

Tests created to validate scenarios of:

• Tested with valid units as enum values.
• Tested with valid units as string.
• Verified that return is string type.
• Tested Invalid unit scenarios to always raise InvalidUnitOfMeasureException

[cfulton]

This change should cover issues raised in:

• Sensor init fails if unit of measurement is a string #326
• validate_unit doesn't allow valid values #327
• [Device Support Request] Tuya ultrasonic level sensor (TS0601 by _TZE284_kyyu8rbj) zha-device-handlers#3356
• Fix missing unit in _TZE200_pay2byax quirk zha-device-handlers#3563
• Add entities for Schneider Electric Thermostats zha-device-handlers#3575

[TheJulianJES]

We've gone with this approach for now: #352
Still, thank you very much for the PR!