Support stale read through SQL hints

## Feature Request

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

Stale read could be enabled by `tidb_read_staleness`, but it has some inconveniences, including:
- Using stale read `tidb_read_staleness` through session-level settings is very complicated, and the users may need to ensure that the session variable is set back after each use of stale read
- It also does not accept adding AS OF TIMESTAMP syntax to SQL to use stale read, which will cause the code to violate common coding rules and be bound to a certain feature of TiDB.

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

Introduce a new HINT to specify the tidb_read_staleness, makes TiDB specify the staleness in the statement, like
```
SELECT /*+ READ_STALENESS(-5) */ * FROM table1 WHERE ...
```
Or together with `SetVar`,
to support enabling query level stale read.

**Describe alternatives you've considered:**

As mentioned above for the usages of `tidb_read_staleness` and `AS OF TIMESTAMP`.

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