[Feature][MTMV] Support modifying `replication_allocation` property for Async Materialized View (MTMV)

### Search prior failures

- [X] I have searched the existing issues and found no similar proposal.

### Is your feature request related to a problem? Please describe.

Currently, when creating an Async Materialized View (MTMV) in Doris, the physical table associated with the MTMV inherits the `replication_allocation` property from the base tables or falls back to global default configurations.

However, in production scenarios with multi-tag BE clusters (e.g., hot/cold data isolation, `default` vs. `offline` tags), users often need to alter the replica allocation of existing Async Materialized Views (e.g., moving MTMV replicas to dedicated `offline` BE nodes to isolate query workloads).

Currently, attempting to modify the `replication_allocation` for an existing MTMV via SQL either lacks dedicated DDL syntax or fails to propagate the property change down to the underlying partitions/table correctly.

### Describe the solution you'd like

1. **Syntax Support**:
Support altering the `replication_allocation` property for Async Materialized Views using DDL, for example:
```sql
ALTER MATERIALIZED VIEW [db_name.]mv_name SET ("replication_allocation" = "tag.location.default: 1, tag.location.offline: 2");

Start by reviewing the existing Async Materialized View DDL and property-handling paths in Doris, then trace how replication allocation is inherited and applied to the underlying physical table and partitions. Done means a supported ALTER MATERIALIZED VIEW syntax changes the requested allocation for existing MTMVs and propagates it correctly.

Written by the indexing model from the issue text.