Wrap local calls to the content and lease service

[rumpl]

- What I did

Fixed the issue where the calls to the content store or the leases manager
would fail because the context doesn't define a namespace.

Wrapped all the calls to the local content and lease services to add
the default namespace in the context.

The wrapper sets the default namespace in the context if none is
provided, this is needed because we are calling these services directly
and not trough GRPC that has an interceptor to set the default namespace
to all calls.

The error is hidden currently because the manifestMatchesPlatform doesn't
return an error if there is one, this PR changes that and now returns an error.
See this comment to see the error that was previously hidden.

- How I did it

Implemented content.Store and leases.Manager.

- How to verify it

- Description for the changelog

- A picture of a cute animal (not mandatory but encouraged)

[thaJeztah]

Looks like this test-helper avoided this case by setting a namespace on the context;

moby/daemon/images/store_test.go

Lines 43 to 44 in df66487

	ctx := namespaces.WithNamespace(context.Background(), t.Name())
	images := &imageStoreWithLease{Store: is, ns: t.Name(), leases: metadata.NewLeaseManager(mdb)}

Slightly confused by that code though;

• imageStoreWithLease has a ns property (so a default namespace)
• but that test-utility ALSO sets the namespace on the context (why both?)

Haven't fully wrapped my head around it, but wondering now if that test is correct or also should be switching to the wrapped one (depends on what we're testing there I guess). Slight complication in that case is that the wrapped store is in the daemon/ package, not daemon/images

[rumpl]

There are two ways we are calling containerd:

• using the client over GRPC, in this case there is a GRPC interceptor that sets the default namespace if the current context doesn't have any.
• directly by creating local services, in this case there is nothing setting the default namespace, my change fixes this.

I'm not sure about the test but in any case the manifestMatchesPlatform uses a context.TODO() so nothing you would do in the test would set the namespace since there are some cases where this call calls the leases directly and not over GRPC, see this comment

[thaJeztah]

Both good suggestions, @corhere!

@rumpl wdyt? worth updating before merging?

[rumpl]

@thaJeztah yeah, I'll update 👍

[thaJeztah]

LGTM, thanks!