feat: ion-datetime: default value when opened

# Feature Request

**Ionic version:**

[x] **4.x**

**Describe the Feature Request**

when opening the `ion-datetime` component there is no directive available to set a default date _when_ opened. Currently, you can set a `value` but that's not expected. I have a section in a registration that has the birthday selection that is not required (not my decision to make it optional). Anyways, the powers that be don't like how when the datetime component is opened the default date is today. And since it is February and there are not 31 days in the month the powers that be wish that we default the month to January `x` year where they would be 18+.

**Describe Preferred Solution**

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**Describe Alternatives**

**Related Code**

**Additional Context**