fix: add generic to LabelFunction to prevent type error for custom translation keys (#9335)

When creating custom translations and merging the custom translation
keys with the default translation keys, and then pass it to the generic
TFunction type, Typescript complains that the function does not satisfy
the LabelFunction type in a label field.

The reason for this is that the LabelFunction type is not generic, and
it's always using the default TFunction which itself uses the
DefaultTranslationKey if no type is passed to it.

This is solved by making the LabelFunction generic and forward the
TTranslationKeys to the TFunction type.

Following this documentation:
https://payloadcms.com/docs/configuration/i18n#typescript 

Example:

![image](https://github.com/user-attachments/assets/583ae9ca-63e7-4f49-9ded-0fca0a4a3d80)

packages/payload/src/config/types.ts

@@ -1,4 +1,5 @@
 import type {
+  DefaultTranslationKeys,
   DefaultTranslationsObject,
   I18nClient,
   I18nOptions,
@@ -493,7 +494,11 @@ export type LocalizationConfig = Prettify<
   LocalizationConfigWithLabels | LocalizationConfigWithNoLabels
 >
 
-export type LabelFunction = ({ t }: { t: TFunction }) => string
+export type LabelFunction<TTranslationKeys = DefaultTranslationKeys> = ({
+  t,
+}: {
+  t: TFunction<TTranslationKeys>
+}) => string
 
 export type StaticLabel = Record<string, string> | string