xds: Use ptypes.MarshalAny() instead of initializing an `any` proto as a struct literal

[easwars]

There are a few places in the xds tests code where we initialize any protos as struct literals. We should instead be using ptypes.MarshshalAny().

This came up during a code review: #4222 (comment)

[Juneezee]

ptypes.MarshalAny is deprecated, we should use anypb.New instead.

Reference: https://pkg.go.dev/github.com/golang/protobuf/ptypes#MarshalAny

[zasweq]

We have MarshalAny() in internal/testutils. See https://pkg.go.dev/google.golang.org/grpc/internal/testutils@master#MarshalAny.

[zasweq]

We'll use that throughout the code.