Add vendored deps

vendor/git.autistici.org/ai3/go-common/clientutil/backend.go

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

vendor/git.autistici.org/ai3/go-common/clientutil/retry.go

vendor/git.autistici.org/ai3/go-common/clientutil/tls.go

...r/git.autistici.org/ai3/go-common/clientutil/transport.go

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

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

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

...t.autistici.org/ai3/go-common/serverutil/load_shedding.go

...t.autistici.org/ai3/go-common/serverutil/proxy_headers.go

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

vendor/github.com/beorn7/perks/LICENSE

vendor/github.com/beorn7/perks/quantile/exampledata.txt

vendor/github.com/beorn7/perks/quantile/stream.go

vendor/github.com/cenkalti/backoff/LICENSE

vendor/github.com/cenkalti/backoff/README.md

vendor/github.com/cenkalti/backoff/backoff.go

vendor/github.com/cenkalti/backoff/context.go

vendor/github.com/cenkalti/backoff/exponential.go

vendor/github.com/cenkalti/backoff/retry.go