Date-Time-Input: support additional time input formats

[ddaribo]

Add the following additional input formats for the Date-Time-Input. Related to this issue and changes from the igniteui-angular repo. Update the spec section about Custom input format.

Format	Description
S	Fractional seconds 1 digit, will be coerced with leading zero while editing.
SS	Fractional seconds 2 digits, will be coerced with leading zero while editing.
SSS	Fractional seconds 3 digits with explicitly set leading zero.
a	AM/PM section for 12-hour format.
aa	AM/PM section for 12-hour format.
aaa	AM/PM section for 12-hour format.
aaaa	AM/PM section for 12-hour format.
aaaaa	a/p section for 12-hour format.

Note: if fractional seconds are to be supported for input, they also have to be supported for display, which isn't currently the case for the IgcDateTimeInput. This is different than the Angular feature, where the display format is handled by the Date Pipe and no additional handling is needed there in that regard.