add --header flag to src auth token to print auth header

[burmudar]

• for access token print Authorization: token <token>
• for oauth token print Authorization: Bearer <token>

Closes CPL-275

Test plan

❯ export SRC_ENDPOINT=https://sourcegraph.sourcegraph.com; go run ./cmd/src/... auth token --header
Authorization: Bearer sgo_at_DEADBEEF

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