Pass the image layers as a Vector

[lswith]

This PR is the result of a long winding issue when calling the client.push function from
within tokio::spawn.

The specific error was:

error: implementation of `Iterator` is not general enough
  --> src/tester.rs:54:5
   |
54 |     res
   |     ^^^ implementation of `Iterator` is not general enough
   |
   = note: `Iterator` would have to be implemented for the type `std::slice::Iter<'0, ImageLayer>`, for any lifetime `'0`...
   = note: ...but `Iterator` is actually implemented for the type `std::slice::Iter<'1, ImageLayer>`, for some specific lifetime `'1`

and

error: implementation of `FnOnce` is not general enough
  --> src/tester.rs:54:5
   |
54 |     res
   |     ^^^ implementation of `FnOnce` is not general enough
   |
   = note: closure with signature `fn(&'0 ImageLayer) -> [async block@Client::push::{closure#0}::{closure#0}::{closure#0}]` must implement `FnOnce<(&ImageLayer,)>`, for any lifetime `'0`...
   = note: ...but it actually implements `FnOnce<(&ImageLayer,)>

or

error: higher-ranked lifetime error
   --> src/client.rs:92:5
    |
92  | /     async move {
93  | |         if *auth != RegistryAuth::Anonymous {
94  | |             client
95  | |                 .auth(&image, auth, oci_distribution::RegistryOperation::Push)
...   |
99  | |         client.push(&image, &layers, config, auth, manifest).await
100 | |     }
    | |_____^

warning: `registry-load-tester` (lib) generated 2 warnings
error: could not compile `registry-load-tester` due to previous error; 2 warnings emitted

Only once I found this specific rust issue: rust-lang/rust#89976 did it make sense that
this was the issue.

[lswith]

This is just a clone of the descriptors, not the actual layers.

[flavio]

Can you show a snippet of the code under tester.rs that triggered the error?

[lswith]

I can't seem to find the code or replicate so I'm happy to remove this PR for now and if it comes back, I'll make a better example.