Add support for partial updates

[anttip]

Adds support for binding to java.util.Optional parameters. Binding to Optional parameters allows implementing partial updates. With a partial update the mutate call can update only some of the Optional fields without needing to repeat all the existing values of an object.

The mapping from an optional field in GraphQL schema that binds to java.util.Optional is the following:

1. value given to the field in GraphQL call is mapped into java.util.Optional of the underlying type
2. null given to the field in GraphQL call is mapped into Optional.empty to signal the removal of a possibly existing value of the field
3. field not included in the GraphQL call is mapped into null to signal undefined so any possibly existing value(s) should not be modified

[Fgerthoffert]

Thank you for the PR @anttip,

After an initial look, this seems like a good idea, things are a bit busy on our end but we'll try to review that PR at some point in May (or earlier if we can).