[Feature] Support CREATE TABLE IF NOT EXISTS

### Search before asking

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

### Motivation

When I use CREATE TABLE IF NOT EXISTS to create a table, if the table already exists, it will report an AlreadyExistsException error. Is this not very friendly? Can it support this syntax like hive?

### Solution

_No response_

### Anything else?

_No response_

### Are you willing to submit a PR?

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

No file, test, or entry point is named in the issue. Start by locating the CREATE TABLE handling and the existing AlreadyExistsException behavior, then trace how SQL syntax reaches table creation. Done means CREATE TABLE IF NOT EXISTS succeeds without raising that exception when the table already exists.

Written by the indexing model from the issue text.