Support batch operation for creating/dropping binding by specifying digests

[time-and-fate]

Enhancement

TiDB supports two kinds of statements:

create [session | global] binding from history using plan digest stringLiteral
drop [session | global] binding for sql digest stringLiteral

Currently, the user can only specify one digest in a statement. We will enhance it so the user can specify multiple digests by using string literal or user variable.

[time-and-fate]

Tests

Creating and dropping bindings can work correctly as in previous single digest cases in the cases below.

• Mix string literals and user variables together.
• Empty strings in the input should be ignored.
• When creating bindings, if multiple input plan digests correspond to the same SQL digest, then only the first one should be effective, the others should be ignored and warnings should be output.
• Atomic: if creating/dropping for one of the input digests fails, the entire SQL should fail.