Support type coercion between timestamp and unixtime

### Is your feature request related to a problem or challenge?

Say there's a table consisting of a timestamp column with the precision seconds. If we can support type coercion between timestamp and unixtime, a query `SELECT * FROM tbl WHERE ts > 1723527660` would be regarded legal and there's no need to wrap the unixtime with the function `from_unixtime`.

### Describe the solution you'd like

Add a cast path between timestamp and unixtime, and try to cast a integer to a certain timestamp type when building a plan. Reports an error when such a cast fails.

### Describe alternatives you've considered

_No response_

### Additional context

_No response_

Start by tracing the type-coercion cast path and the plan-building step described in the issue. Define how timestamp and unixtime values are coerced, including integer-to-timestamp planning, then verify that invalid casts produce an error and that the shown timestamp comparison is accepted.

Written by the indexing model from the issue text.