types(index): allow undefined for some optional properties

[Fdawgs]

This PR updates the types of some optional properties to be undefined, because they check against undefined with nullish coalescing operators and assign defaults:

fastify-basic-auth/index.js

Lines 73 to 74 in e1e72e2

	const strictCredentials = opts.strictCredentials ?? true
	const useUtf8 = opts.utf8 ?? true

Checklist

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