Add support for setting `timezone` on DateTimePicker input

Reproducible in:

The Slack SDK version

"slack/bolt": "^3.12.2",
"slack/web-api": "^6.8.1"

or refer to your package.json

Node.js runtime version

node v16.14.0

OS info

ProductName:		macOS
ProductVersion:		13.2
BuildVersion:		22D49

Steps to reproduce:

As shown in the picture below, I have set a specific timezone in my Slack App settings.

My OS timezone is:

❯ sudo systemsetup -gettimezone
Time Zone: Europe/Dublin

When using the DateTimePicker input, the timezone displayed under the time-picker differs from what we have set in the User Preferences.

Because of this, the input's initial_date_time value is wrong and not respecting Slack's user-preferred timezone.

Also, the DateTimePicker input timezone field cannot be set/unset and it's always displayed - this does not happen for a Timepicker instance

Expected result:

The DateTimePicker should use the User Preferences' timezone, instead of the OS's one.

Actual result:

The DateTimePicker parses the initial_date_time value using the wrong timezone.

Start by reproducing the DateTimePicker case with the listed Slack SDK versions and inspect the TypeScript handling of the DateTimePicker input, especially initial_date_time and timezone. Done means the input uses the Slack user's preferred timezone and allows the timezone field to be set or unset consistently with Timepicker.