extstore should use a dedicated storage URI sysvar

### Problem
`extstore` currently reads `tidb_cloud_storage_uri`, which is also used by DDL add index and import related flows. This couples extstore storage selection with unrelated features and makes it hard to configure extstore independently.

### Proposed change
Add a dedicated global system variable, `tidb_external_storage_uri`, for extstore.

### Expected behavior
- `extstore` reads `tidb_external_storage_uri` instead of `tidb_cloud_storage_uri`
- the new variable keeps the same URI validation and redaction behavior as `tidb_cloud_storage_uri`
- `tidb_cloud_storage_uri` remains available for its existing users

### Affected area
- extstore
- sysvars / session variable plumbing
- redaction / invisibility handling