planner: allow `tidb_request_source_type` and `tidb_low_resolution_tso` in `SET_VAR` hints

## Feature Request

**Is your feature request related to a problem? Please describe:**

`tidb_request_source_type` and `tidb_low_resolution_tso` can only be changed with `SET`, so they apply to the whole session. Applications that share connections (for example through a connection pool or a proxy) cannot safely tag a single query with a request source or let a single query read with a low-resolution TSO without also changing the behavior of every following statement on that connection.

**Describe the feature you'd like:**

Allow both variables in the `SET_VAR` optimizer hint so they can be scoped to one statement, the same way `MAX_EXECUTION_TIME` and `RESOURCE_GROUP` already can:

```sql
SELECT /*+ SET_VAR(tidb_request_source_type='lightning') */ ... ;
SELECT /*+ SET_VAR(tidb_low_resolution_tso=ON) */ ... ;
```

The hinted value should also reach an already active transaction (the request source is copied into the transaction when it is activated) and be restored after the statement finishes.

**Describe alternatives you've considered:**

Wrapping every such query in `SET @@tidb_request_source_type = ...; ; SET @@tidb_request_source_type = ;`. This costs extra round trips and leaks the setting to other statements if the middle statement fails or the connection is returned to the pool early.

**Teachability, Documentation, Adoption, Migration Strategy:**

Documentation only needs to list the two variables as `SET_VAR`-capable in the system variable reference. No migration is required since existing `SET` usage keeps working.

Start by tracing the existing SET_VAR handling for MAX_EXECUTION_TIME and RESOURCE_GROUP, then inspect how tidb_request_source_type and tidb_low_resolution_tso are applied to session and active-transaction state. Add coverage for both hinted variables, including restoration after the statement, and update the system variable reference to list their SET_VAR support.

Written by the indexing model from the issue text.