Allow function as valid type for UrlSchemaOptions.customFetch

[ArminWiebigke]

Description

This fixes the type definition of UrlSchemaOptions.customFetch, to match the actual values that are allowed to be used: Allow a function to be provided as customFetch.

See #9667

Type of change

• Bug fix (non-breaking change which fixes an issue)
• This change requires a documentation update

How Has This Been Tested?

Added a new test that uses a function as value for customFetch, although the actual implementation is in graphql-tools.

Checklist:

• I have followed the CONTRIBUTING doc and the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[changeset-bot]

🦋 Changeset detected

Latest commit: 7982799

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[ArminWiebigke]

Can anyone take a look at this?