Support generating a VLAN-tagged Ethernet frame

## Background

Currently, we can read Ethernet frames with VLAN-tags, but not generate/create ones. We'd like to be able to generate packets with VLAN-tagged Ethernet frames. More information about tags can be found at https://docs.rs/capsule/latest/capsule/packets/struct.Ethernet.html#8021q and https://docs.rs/capsule/latest/capsule/packets/struct.Ethernet.html#8021ad, including links to specs/Wikipedia.

## Notes on Addition

[VlanTag](https://docs.rs/capsule/latest/capsule/packets/struct.VlanTag.html) methods will need setters for these fields.

Start with the Ethernet and VlanTag documentation linked in the issue, then inspect the existing VLAN-tag parsing and Ethernet frame-generation paths. Determine how VlanTag fields are represented and where setters are needed. Done means callers can generate Ethernet frames with 802.1Q and 802.1ad VLAN tags, matching the documented tag formats.

Written by the indexing model from the issue text.