fix: Allow readonly string[] for Subscription topics

[robhogan]

I just ran into this while attempting to pass something like ['Topic1', 'Topic2'] as const to subscription options - there's no reason to restrict to mutable arrays here.

[MichalLytek]

Please try to reproduce the error in tests, push the failing commit and then after CI fail push the fixing commit.

[robhogan]

Ach, looks like the issue starts upstream in graphql-subscriptions, (apollographql/graphql-subscriptions#234) where the change would be breaking.

Thanks for the quick response anyway, closing.