Implement to_timestamp_millis etc #355

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This is a follow up from the discussion in this PR:
apache/arrow#10005

Namely for first class casting to non nanosecond timestamp types and functions supporting those.

There was also a email thread on the list about this.

Describe the solution you'd like
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The proposal is as follows:

• A function to_timestamp_millis() which converts from the following types:
  • String - converts timestamp strings, just like to_timestamp() except to a TimestampArray with Milliseconds
  • Other TimestampArray's - casts them with truncation as necessary to Millisecond resolution
  • Int64 - assumes the values are already millisecond resolution and just does a cast to TimestampArray with Millis
• Similar functions to_timestamp_micros() and to_timestamp_seconds()

Describe alternatives you've considered
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We considered in the above threads to reuse datetime_trunc() for this purpose.

The issues with using datetime_trunc() are as follows:

• Datetime trunc is primarily used for truncating an existing timestamp to say millis, seconds, hours, minutes, etc., and not for changing the type of the date time.
• Without modifying the existing signature it would not be possible to disambiguate between, for example, using datetime_trunc to truncate to seconds but keeping nanosecond resolution, or actually casting to Timestamp type with Seconds resolution
• Additional infrastructure is needed in the function evaluation code to support evaluating text/constant arguments in function type output determination, right now the output type is determined based on the input types alone

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