[BUG] Fix issues #64 #65 #66

[nicklambourne]

Change Summary

• Addresses the following issues:
  • DatePicker errors when not given optional inital_date on creation. #66
    • Adds a _resolve-time check to omit the field from the produced dictionary when initial_date is not provided, as suggested.
  • DateTimePicker not in ALLOWED_INPUT_ELEMENTS for InputBlock #65
    • Adds DateTimePicker to ALLOWED_INPUT_ELEMENTS as suggested.
  • StaticSelectMenu & StaticMultiSelectMenu do not support default Option TextType #64
    • Adds creation-time validation to ensure that Options passed to StaticSelectMenu and StaticMultiSelectMenu use TextType.PLAINTEXT as suggestd.

CC: @K8sKween