[TP/Const Assertions] Use const assertion technique in Talent Pool types

[helloanil]

Context/background

#412 handles refactoring ReDI Connect and shared types by using const assertion technique.

What needs to be done?

We need to apply the same technique to refactor Talent Pool types.

Some useful links

https://www.notion.so/binarylights/Typescript-type-extraction-typeof-keyof-as-const-a6d576da58ac447ea397988c119c2cf9 https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-4.html#const-assertions