Add go.mod, update dependencies, build on Bullseye

.gitlab-ci.yml

debian/control

debian/rules

go.mod

go.sum

vendor/contrib.go.opencensus.io/exporter/zipkin/.travis.yml

vendor/git.autistici.org/ai3/go-common/.gitlab-ci.yml

vendor/git.autistici.org/ai3/go-common/go.mod

vendor/git.autistici.org/ai3/go-common/go.sum

vendor/git.autistici.org/ai3/go-common/renovate.json

vendor/git.autistici.org/ai3/go-common/serverutil/http.go

vendor/git.autistici.org/id/auth/.gitignore

vendor/git.autistici.org/id/auth/.gitlab-ci.yml

vendor/git.autistici.org/id/auth/README.md

vendor/git.autistici.org/id/auth/protocol.go

vendor/git.autistici.org/id/keystore/.gitlab-ci.yml

vendor/git.autistici.org/id/keystore/README.md

vendor/github.com/beevik/etree/.travis.yml

vendor/github.com/cenkalti/backoff/.gitignore

vendor/github.com/cenkalti/backoff/.travis.yml