Move containerd media type warnings for attestations

[jedevc]

⬆️ Follow-up to #3063

This moves the containerd suppressions for media type warnings from the
exporter package into the push and copy packages which allow them to be
used by all lower-level buildkit utilities, e.g. tests:

buildkit/util/testutil/integration/run.go

Lines 256 to 258 in 81d19ad

	if err := contentutil.CopyChain(context.TODO(), ingester, provider, desc); err != nil {
	return err
	}

.

Previously, this would cause containerd media type warnings to be printed in test logs if an image with attestations was copied into the local image cache.

While working on this, I also noticed that we can remove our custom definition of the application/vnd.in-toto+json media type and replace it with intoto.PayloadType (which we were already using in some places, but inconsistently).

[tonistiigi]

Although you want to be precise is here and only change the context where it is needed it is better if this is always the first line of the function because the call is not specific to "intoto" at all. Also add a comment where this is used that this registers types that are not defined by default. Otherwise this reads like push.Push() or contentutil.Copy always register current payloadtype as "intoto".

Maybe this is better in a util pkg function, eg. RegisterContentPayloadTypes()

[jedevc]

Although you want to be precise is here and only change the context where it is needed it is better if this is always the first line of the function because the call is not specific to "intoto" at all

Makes sense, it's a lot cleaner all at the top 😄

Maybe this is better in a util pkg function, eg. RegisterContentPayloadTypes()

Agreed, have moved this to contentutil.RegisterContentPayloadTypes().