feat: generalize continuity results for the continuous functional calculus (#30594)

Under suitable conditions `x ↦ cfc f (a x)` is continuous. Previously these conditions were that there was a single compact set on which `f` was continuous, and which contained the spectra of all `a x`. However, this is a bit too stringent for some purposes and so we generalize it here. In particular, it suffices that the spectra `a x` are covered by a family of compact sets `s x` (on each of which `f` is continuous) satisfying the property that each `s x₀` contains the spectra `a x` for `x` sufficiently close to `x₀`.

Author: j-loreaux