Release v2.18.0

Added

• The sign-in notification can carry a signed envelope binding its origin, audience, method and target, a digest of the exact bytes posted, its issue time and a single-use identifier, so an application can verify that a sign-in it is told about really came from the auth proxy (#82)
• SignIn:Attestation configuration, with rotating signing keys selected by key identifier, enabling the signed envelope (#82)

Security

• A configured sign-in notification that cannot be signed is no longer sent at all, rather than falling back to an unsigned call (#82)
• Signing refuses an RSA key smaller than 2048 bits (#82)
• A sign-in notification endpoint carrying a query string is rejected at startup, because the route binding deliberately excludes the query and would otherwise leave it unprotected (#82)
• The attestation signing contract no longer renders its private key when written as text (#82)