Specify dateTime field as human readable string

Hello,

Following a discussion on [this thread](https://apache-pinot.slack.com/archives/C011C9JHN7R/p1622466597303800) I would like to propose an enhancement wherein a pinot operator can define a dateTimeField in a schema and have its result being shown to the Pinot user at query time as a human readable string while at the same time being able to use date time transformations as mentioned [here](https://docs.pinot.apache.org/users/user-guide-query/supported-transformations#datetime-functions).

As an example:
I.e: Showing to the user a date time field in the format: yyyy-MM-dd on which they can do `year(field)` or `month(field)` transformations
Where a query like: `select year(dateBirth), dateBirth from table where...`
outputs: `1982 | 1982-02-13`

Thank you.

No source file, test, or entry point is named. Start by tracing schema dateTimeField handling and the query date-time transformation documentation linked in the issue; done means a configured field is displayed as a human-readable string while year(field) and month(field) transformations continue to work as shown in the example.

Written by the indexing model from the issue text.