Pass SDK options down as `readonly`

[lforst]

As discussed with @timfish in https://github.com/getsentry/sentry-javascript/pull/5203#issuecomment-1145759085, we were contemplating whether it would be a good idea for the SDK to pass options down as readonly. In that particular case it would have caught a bug, and might also do so in the future.

This is non-trivial of course, so we can just use this issue as grounds for discussion before actually starting to implement this.

(Just a random thought: This could be done as a hacktober PR)