[packet] Introduce context type parameter to Serializer

Introduces:

* a new `SerializationContext` trait
* a concrete no-op implementation of the trait
* a context parameter on `Serializer`
* a context argument on every serialization method on `Serializer`
* `SerializerExt`, split from `Serializer` to defer parameter resolution

and then updates all the downstream code so that it builds.

Note: this CL does not introduce any logic changes; serialization methods
are always called with `NoOpSerializationContext`, and `Serializer`
implementations do nothing with the context save for passing it down to
wrapped serializers where applicable.

Netstack3 also gains a new `NetworkSerializer` trait that's intended to
abstract away the concrete serialization context type used by NS3. A
future CL will add `NetworkSerializationContext` and switch the trait
over.

Bug: 485599557
Change-Id: I2d8acf1713fb6766985e0b3db95b69fb6a6a6964
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1600008
Reviewed-by: Antonio Linhart <antoniolinhart@google.com>
Reviewed-by: Andrew Davies <awdavies@google.com>
Reviewed-by: Jeff Martin <martinjeffrey@google.com>
Reviewed-by: Justin Mattson <jmatt@google.com>
Reviewed-by: Zeling Feng <zeling@google.com>
Fuchsia-Auto-Submit: Eric Rinehart <erinehart@google.com>
Commit-Queue: Eric Rinehart <erinehart@google.com>