plugin: Support Pre-parse Query Rewrite for Audit plugins

[kennytm]

Feature Request

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

In MySQL, Pre-parse Query rewrite plugin is the way to provide custom SQL syntax (example). Query rewrite plugin is a subtype of Audit plugin, and TiDB does support an OnParseEvent hook. Unfortunately OnParseEvent is not invoked anywhere, and is not in the correct shape to allow actual query rewriting.

Describe the feature you'd like:

Remove OnParseEvent, and introduce a RewriteSQL field for rewriting the query before parsing.

struct {
    RewriteSQL func(
        ctx context.Context, 
        sctx *variable.SessionVars, 
        sql string,
    ) (string, error)
}

The RewriteSQL for each plugin should be invoked inside session.Parse() before calling parser.New().

RewriteSQL should be expected to run for a long time (e.g. more than 1 week) as long as ctx is not done.

It is unspecified what should happen if multiple Query Rewrite plugins coexist.

Describe alternatives you've considered:

None.

If we want to support Post-parse Query rewrite plugins too, it could be named RewriteAST accepting those []ast.StmtNode.

Teachability, Documentation, Adoption, Migration Strategy:

[zz-jason]

@lysu PTAL