Set `VENDOR_NAME` in `/etc/os-release` and make it configurable via Twoliter.toml

## What I'd like

`VENDOR_NAME` is a field in `/etc/os-release` that provides extra metadata about the producer of the OS:
> The name of the OS vendor. This is the name of the organization or company which produces the OS. This field is optional.

https://www.freedesktop.org/software/systemd/man/latest/os-release.html#VENDOR_NAME=

I would like for builders of Bottlerocket to be able to set this field via the Twoliter.toml project file. The intention of setting a `VENDOR_NAME` is to allow for Out-of-Tree builders of Bottlerocket projects to distinguish a custom variant of Bottlerocket from a variant maintained by the bottlerocket-os org.

Start by locating how Twoliter.toml project settings are parsed and where Bottlerocket's /etc/os-release content is generated. Trace the existing metadata fields before deciding where this setting belongs. Done means a configured VENDOR_NAME is emitted in /etc/os-release for custom builds, with coverage for the configuration path if the project has corresponding tests.

Written by the indexing model from the issue text.