improve information at RequestContext for rich logging

[vvakame]

I want to get operationName and persistedQueryHash to logging in handler.RequestMiddleware.

Currently, We can't get accurate information.

for example, We can't detect which operation is in execute from RequestContext.
https://github.com/99designs/gqlgen-contrib/blob/c654377d611cfcf5b77012b5814af5a14a79bc86/gqlopencensus/tracer.go#L136-L147

query A {
  a: viewer {
    email
  }
}

query B {
  b: viewer {
    email
  }
}

Current tracer implementations can't get operationName B when it chosen.

so, I think func NewRequestContext(doc *ast.QueryDocument, query string, variables map[string]interface{}) *RequestContext is not extensible without breaking change.
I suggest to make func (rctx *RequestContext) Validate() error to check and fill empty field and deprecate NewRequestContext.

What do you think about my suggestion? ✨

[vektah]

Sounds good to me