[Feature] Support hidden partition like Iceberg

### Search before asking

- [X] I searched in the [issues](https://github.com/apache/paimon/issues) and found nothing similar.

### Motivation

Iceberg's hidden partition is easy to use which prevents user mistakes that cause silently incorrect results or extremely slow queries.
And partition key can be transformed from existed column.

### Solution

_No response_

### Anything else?

_No response_

### Are you willing to submit a PR?

- [X] I'm willing to submit a PR!

No source files, tests, or entry points are named in the issue. Start by reviewing how Paimon currently defines partition keys and compare that behavior with Iceberg's hidden partition concept; done means the requested hidden-partition support is specified and implemented with coverage for transforming an existing column.

Written by the indexing model from the issue text.