[graphiql] Add ability for custom namespace for graphiql variables stored in localstorage

Hi, I have deployed multiple service in same domain with path based approach (due to project requirement).

Example:
1. `https://api.domain.com/serviceAbc/graphiql`
2. `https://api.domain.com/serviceXyz/graphiql`

As localStorage is used for `https://api.domain.com` as a whole and graphiql uses the constant `graphiql:` as prefix, the values will be overwritten for both services.

Please allow to set a optional custom storage namespace so that the values can be stored with `{{namespace}}:graphiql:` prefix.

Note: I am using cdn based approach in graphiql