Migrate to Lab Vault Cert for E2E Tests (#7085)

This PR:
- Adds `3p-e2e.yml` which defines a OneBranch pipeline to run 3P MSAL JS
E2E tests to replace existing GitHub workflows
- Updates `gen_env.sh` to use a new PowerShell script called
`gen_env.ps1` that creates and populates a .env file with E2E Test
environment variables (Express session secret, ClientID, TenantID and
the absolute path of the client certificate used to authenticate with
the Lab App)
- Updates `e2e-test-utils/LabClient` to use a `CertificateCredential`
instead of `ClientSecretCredential`
- Updates the `b2c-user-flows` Node sample to use a SESSION_SECRET env
variable instead of the AZURE_CLIENT_SECRET