codegen doesn't properly handle postponed type annotations

[jbkoh]

strawberry codegen --schema schema.graphql generates models referencing postponed types as its class variable instead of using a string to be deferred.

Describe the Bug

From schema, the following code has been generated. In this example, AccessCredentialFilter is referencing itself inside the model

@strawberry.input
class AccessCredentialFilter:
    and_: list[AccessCredentialFilter | None] | None = strawberry.field(name="and")

However, it should rather be

@strawberry.input
class AccessCredentialFilter:
    and_: list[Union["AccessCredentialFilter", None]] | None = strawberry.field(name="and")

or

@strawberry.input
class AccessCredentialFilter:
    and_: list[Optional["AccessCredentialFilter"]] | None = strawberry.field(name="and")

System Information

• Operating system: Ubuntu 22.04
• Strawberry version (if applicable): 0.248.1

Additional Context

It seems there was a related discussion: #769
but not sure if it was discussed in the context of codegen.

I'd be happy to contribute if I can be directed to the right part of the code.

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