Calendar: Declarative way of setting the selected days

[ilhan007]

Is your feature request related to a problem? Please describe.
The Calendar's selectedDates property works only with array of timestamps like calComponent.selectedDates = [1600041600]. But, this is not convenient when the Calendar is used inside a framework, that works with templates (like React) as the selectedDates can't be set in declarative way.

Note: The requirement origins from #2528.

Describe the solution you'd like
To propose and implement API to support declarative way of setting the selected days in the ui5-calendar,
that fits for all 3 types of selection - Single date, Multiple dates and Range of dates.

Prerequisites
The API proposal should be synced with the WCC team, before implementation starts.

Describe alternatives you've considered
Here are are some API proposals that we considered in the past without coming to a final agreement.
These proposals are just FYI. Your API proposal can be something completely different, or combination of the ones below.

• value property (same as we have in the DatePicker - works perfect for Single date selection:

<ui5-calendar value="11/11/2018">

• firstValue, lastValue properties - works great for Range selection scenario:

<ui5-calendar firstValue="11/11/2018" lastValue-"20/11/2018">

• slot and dedicated element - can cover the Multiple selection scenario:

<ui5-calendar>
   <ui5-calendar-date value="11/11/2018"></ui5-calendar-date>
   <ui5-calendar-date value="20/11/2018"></ui5-calendar-date>
  <ui5-calendar-date value="06/12/2018"></ui5-calendar-date>
</ui5-calendar>

[ilhan007]

Originated from: #1730