Allow config dump to generate to a file directly instead of stdout #2146

[dklimpel]

It is a little bit copy&paste from

• Allow config generate to generate to a file directly instead of stdout #2146

Writing the dump to the file system is only consequent.

I am not very familiar with rust to bring it in a seperate function for deduplicated code.

[dklimpel]

I am not sure if the failing pipeline is my fault:

error[vulnerability]: Some Ref methods are unsound with some type parameters
    ┌─ /github/workspace/Cargo.lock:[54](https://github.com/matrix-org/matrix-authentication-service/actions/runs/7408315024/job/20156268314?pr=2234#step:4:55)3:1
    │
543 │ zerocopy 0.7.26 registry+https://github.com/rust-lang/crates.io-index
    │ --------------------------------------------------------------------- security vulnerability detected
    │
    = ID: RUSTSEC-2023-0074
    = Advisory: https://rustsec.org/advisories/RUSTSEC-2023-0074
    = The `Ref` methods `into_ref`, `into_mut`, `into_slice`, and `into_slice_mut` are unsound
      and may allow safe code to exhibit undefined behavior when used with `Ref<B, T>` where `B`
      is [`cell::Ref`](https://doc.rust-lang.org/core/cell/struct.Ref.html) or
      [`cell::RefMut`](https://doc.rust-lang.org/core/cell/struct.RefMut.html). Note that these
      methods remain sound when used with `B` types other than `cell::Ref` or `cell::RefMut`.

[sandhose]

Thanks! CI failures was solved in another PR, so I'm going to merge main in your branch to fix it