[Feature] Add expression-based column default values

### Search before asking

- [x] I had searched in the [issues](https://github.com/apache/doris/issues?q=is%3Aissue) and found no similar issues.

### Description

Previously, column DEFAULT values in Doris were primarily treated as literal constants (or a small set of special built-ins such as CURRENT_TIMESTAMP), which limited usability for common patterns like “derive default from current date/time” or “compose a formatted default string”.

### Use case

Example (date/datetime defaults):
d DATEV2 NOT NULL DEFAULT to_date(now())
dt DATETIMEV2(3) NOT NULL DEFAULT now(3)
Example (composed string default):
s STRING NOT NULL DEFAULT concat('a-', DATE_FORMAT(now(), '%M %e, %Y'))

### Related issues

Not yet

### Are you willing to submit PR?

- [x] Yes I am willing to submit a PR!

### Code of Conduct

- [x] I agree to follow this project's [Code of Conduct](https://www.apache.org/foundation/policies/conduct)

The issue names no files or tests. Start by locating Doris’s column DEFAULT handling and the expression functions used in the examples, then trace how date/time and string expressions are validated. Done means the shown date, datetime, and composed-string defaults are accepted and produce the expected values.

Written by the indexing model from the issue text.