fix(server/(config): add config validation for `session_secret` #3452

Job	Run time
grafana-compatibilitycheck (ott-vis-panel)	2m 37s
typos	7s
grafana-compatibilitycheck (ott-vis-datasource)	2m 26s
lint (18.x)	3m 12s
test (18.x)	3m 10s
test (20.x)	3m 16s
Deploy Monolith on fly.io (staging)	0s
Deploy Monolith on fly.io (prod)	0s
Deploy docker image	0s
Deploy Balancer on fly.io (staging)	0s
Deploy Balancer on fly.io (prod)	0s
	14m 48s

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