feat(graphql): add support for transforming the resolvers

[JoshVee]

PR Checklist

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• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
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PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the current behavior?

The GraphQLFactory discovers all the resolvers using the ResolversExplorerService and ScalarsExplorerService. These resolvers are then converted into a schema using makeExecutableSchema().

Issue Number: N/A

What is the new behavior?

This PR introduces a new option on GqlModuleOptions to support transforming the resolvers before creating the schema. This follows a similar pattern to the existing transformSchemas option.

Being able to transform resolvers enables resolver composition, which in our case is needed to migrate from an existing GraphQL implementation to NestJS - eg. https://the-guild.dev/graphql/tools/docs/resolvers-composition.

Does this PR introduce a breaking change?

• Yes
• No

Other information

Resolver composition example

[JoshVee]

@kamilmysliwiec do you see any issues with this approach?

[kamilmysliwiec]

LGTM