Use a union-aware keyof operator

[A5rocks]

Fixes #4063

Based off of https://www.typescriptlang.org/docs/handbook/2/conditional-types.html#distributive-conditional-types.

I believe this still satisfies the initial reason these keyofs were here. As a previous comment on ResolveFastifyRequestContext (the previous generation of this) said:

/**
 * This type handles request context resolution either via generic arguments
 * or type provider. If the user specifies both generic arguments as well as
 * a type provider, this type will override the type provider and use the
 * generic arguments. This enables users to override undesirable inference
 * behaviours in the type provider.
 */

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[mcollina]

lgtm

[mcollina]

Good work!

[A5rocks]

Thanks!

[github-actions]

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