fix: Ensure types allow optional function arguments to be omitted

[zhibek]

Related Issue

The current type definitions do not distinguish between optional and required function arguments.

Therefore, the standard NordigenClient instantiation in Typescript results in a type error:

const client = new NordigenClient({
  secretId: process.env.SECRET_ID,
  secretKey: process.env.SECRET_KEY,
});

$ npm exec tsc
src/example.ts:1:35 - error TS2345: Argument of type '{ secretId: string; secretKey: string; }' is not assignable to parameter of type '{ secretId: string; secretKey: string; baseUrl: string; }'.
  Property 'baseUrl' is missing in type '{ secretId: string; secretKey: string; }' but required in type '{ secretId: string; secretKey: string; baseUrl: string; }'.

Proposed Changes

• Update type definitions for NordigenClient constructor() and initSession() to recognise optional arguments.

Additional Info

n/a

[Betree]

Would be great to merge this one!