fix(mapSchema): allow custom serialization function

[rupert648]

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Description

Checks to see if a custom serialization function has been added during mapSchema, and ensures this new serialization function is added to the rewired type.

Related #5119

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

Screenshots/Sandbox (if appropriate/relevant):

Example repo showing the bug https://github.com/rupert648/graphql-tools-serialization-bug-demo

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• Created a test in mapSchema.test.ts to ensure this functionality is applied

Test Environment:

• OS: macOS Venture 13.2.1
• @graphql-tools/schema : "9.0.13"
• NodeJS: v16.17.0

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 and linter rules pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

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Latest commit: cd0af42

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[ardatan]

See my comment
#5119 (comment)