Custom secretResolver should return unknown/any type?

Use `KeyVaultOptions.secretResolver` to resolve the secret reference with customized logic, e.g. parse JSON string into an object.
```ts
/**
* Specifies the callback used to resolve key vault references that have no applied SecretClient.
* @param keyVaultReference The Key Vault reference to resolve.
* @returns The secret value.
*/
secretResolver?: (keyVaultReference: URL) => string | Promise;
```
Note: Here I think `secretResolver` should return `known` type instead of `string`, as we allow values of the config map to be any type.

_Originally posted by @Eskibear in https://github.com/Azure/AppConfiguration-JavaScriptProvider/issues/78#issuecomment-2287726906_