Support stale read through SQL hints

[cfzjywxk]

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: