Allow coercion between timestamp and int64 types

[maxburke]

Closes #11039

[alamb]

Thanks @maxburke -- I think this PR needs some tests to show the behavior

Perhaps you could add a .slt test

I also wonder if it would be confusing for people to have coercion done automatically

It is also possible to make a int into a timestamp explicitly using one of the to_timestamp functions:

https://datafusion.apache.org/user-guide/sql/scalar_functions.html#to-timestamp

> select arrow_typeof(to_timestamp(12345)), to_timestamp(123456789);
+------------------------------------------+--------------------------------+
| arrow_typeof(to_timestamp(Int64(12345))) | to_timestamp(Int64(123456789)) |
+------------------------------------------+--------------------------------+
| Timestamp(Nanosecond, None)              | 1973-11-29T21:33:09            |
+------------------------------------------+--------------------------------+
1 row(s) fetched.
Elapsed 0.001 seconds.

> select arrow_typeof(to_timestamp(123456789)), to_timestamp(123456789);
+----------------------------------------------+--------------------------------+
| arrow_typeof(to_timestamp(Int64(123456789))) | to_timestamp(Int64(123456789)) |
+----------------------------------------------+--------------------------------+
| Timestamp(Nanosecond, None)                  | 1973-11-29T21:33:09            |
+----------------------------------------------+--------------------------------+
1 row(s) fetched.
Elapsed 0.001 seconds.

> select arrow_typeof(to_timestamp_millis(123456789)), to_timestamp_millis(123456789);
+-----------------------------------------------------+---------------------------------------+
| arrow_typeof(to_timestamp_millis(Int64(123456789))) | to_timestamp_millis(Int64(123456789)) |
+-----------------------------------------------------+---------------------------------------+
| Timestamp(Millisecond, None)                        | 1970-01-02T10:17:36.789               |
+-----------------------------------------------------+---------------------------------------+
1 row(s) fetched.
Elapsed 0.001 seconds.

[alamb]

Marking as draft so it doesn't appear on the "needs review" list while feedback is addressed

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