[ktor server] Ability to specify custom GraphQL server

[dariuszkuc]

Is your feature request related to a problem? Please describe.
Right now, plugin does not have an option to provide custom Ktor GraphQL server which means that plugin cannot be used with custom servers.

Describe the solution you'd like
Ktor GraphQL plugin should allow users to provide custom servers.

Describe alternatives you've considered
N/A

Additional context
N/A