dev/sg: unify ExternalSecrets with secrets store, minor fixes for frontend-e2e

Created by: bobheadxi

• Avoid having 2 sources of secrets by folding secretmanager functionality into secrets.Store, which also enables in-mem caching of these secrets (https://github.com/sourcegraph/sourcegraph/pull/34627/files#r864928821)
• Fix instructions for frontend-e2e

Test plan

go run ./dev/sg test frontend-e2e, see secret getting populated in browser automatically