feat/auth: add src auth token

[burmudar]

This adds the command src auth token, which will print either your access token or your oauth token. This is primarily useful for scenarios like curl -H "Authorization: Bearer $(src auth token)" https://sourcegraph.sourcegraph.com/.api/something.

If no SRC_ACCESS_TOKEN is specified and we use the OAuth token and the token is close to expiring or has expired it will be automatically refreshed

• Centralized the refreshing logic to TokenRefresher which the oauth transport now uses as well as the OAuth token
• Create a constructor NewTransport so that TokenRefresher is properly initialized for the OAuth.Transport

Closes CPL-274

Future work

• support src auth status and tell you if we're using SRC_ACCESS_TOKEN or OAuth and with which endpoint

Test plan

src-cli on  wb/src-auth [$?] via 🐹 v1.25.5
❯ export SRC_ENDPOINT=https://sourcegraph.sourcegraph.com; go run ./cmd/src/... auth token
sgo_at_DEADBEEF

src-cli on  wb/src-auth [$?] via 🐹 v1.25.5 took 2s
❯ export SRC_ENDPOINT=https://sourcegraph.sourcegraph.com; export SRC_ACCESS_TOKEN='hello-reviewer'; go run ./cmd/src/... auth token
hello-reviewer

When there is no OAuth Token and no SRC_ACCESS_TOKEN

export SRC_ENDPOINT=https://sourcegraph.sourcegraph.com; go run ./cmd/src/... auth token
error loading OAuth token; set SRC_ACCESS_TOKEN or run `src login`: secret not found

[eseliger]

Didn't read all the code but it's a great addition!