Can't get correct types extending SchemaDirectiveVisitor

[ilatovsky]

Issue

Couldn't get correct types when i wrote own directive handler.
If i extend from SchemaDirectiveVisitor class MyDirective extends SchemaDirectiveVisitor
this.args is always resolved by typescript as {[name: string]: any;}

Suggestion

Rewrite SchemaDirectiveVisitor as Generic to provide args, context and schema types for inherited classes

[yaacovCR]

Fold into #1306