Flink: Support ALTER TABLE ... DROP PARTITION syntax

### Feature Request / Improvement

## Motivation
Flink already supports ```ALTER TABLE ... DROP PARTITION``` and invokes the catalog's ```dropPartition(...)``` implementation. However, the Iceberg Flink catalog does not currently implement this operation, leaving users without a SQL-level mechanism to remove all data in a partition.

### How users drop a partition today
DELETE is not supported by the Iceberg Flink catalog for batch or streaming. Users must drop into the Java/Scala Table API (table.newDelete().deleteFromRowFilter(...).commit()) or move the operation outside SQL entirely.

## Proposed feature
Add support for the ```ALTER TABLE … DROP PARTITION``` syntax, addressing the partition field name, in the same human-readable form Iceberg already writes to manifest partition paths:
```sql
ALTER TABLE events DROP PARTITION (id = 0);
ALTER TABLE events DROP PARTITION (region = 'eu', dt = '2024-01-01');
ALTER TABLE events DROP IF EXISTS PARTITION (data = 'z')
```
## Implementation sketch

- Flink: ```FlinkCatalog.dropPartition(...)``` translates the partition specification into an Iceberg row filter and invokes ```deleteFromRowFilter(...)```
- Flink's SQL parser already accepts ```ALTER TABLE … DROP [IF EXISTS] PARTITION (...)```.

## Safety
```deleteFromRowFilter``` already runs the strict-projection check at commit, so partial-file deletes are rejected by Iceberg core; the new API surface cannot silently remove rows outside the targeted partition.

### Query engine

Flink

### Willingness to contribute

- [x] I can contribute this improvement/feature independently
- [ ] I would be willing to contribute this improvement/feature with guidance from the Iceberg community
- [ ] I cannot contribute this improvement/feature at this time

Start with FlinkCatalog.dropPartition(...) and the existing catalog partition-operation flow. Trace how the partition specification is converted into an Iceberg row filter and how deleteFromRowFilter(...) is committed; done means the listed ALTER TABLE DROP PARTITION and DROP IF EXISTS examples remove the targeted partitions safely.

Written by the indexing model from the issue text.