fix: [Issue 582] Log successful and unsuccessful sign in attempts

[avinashdhanshetty]

• Added Info and Error logs upon successful and unsuccessful sign-in attempts
• These logs may help for debugging/auditing/security reviews.
• Sample logs as following:

Running tool: /usr/local/go/bin/go test -timeout 30s -run ^TestSessionsCrud$ github.com/crewjam/saml/samlidp
=== RUN   TestSessionsCrud

2025/02/27 19:48:23 User 'alice' authenticated successfully
2025/02/27 19:48:23 ERROR: User 'unknown' doesn't exists
2025/02/27 19:48:23 ERROR: Invalid password for user 'alice'
--- PASS: TestSessionsCrud (0.22s)
PASS
ok      github.com/crewjam/saml/samlidp 0.475s