Add OKTA connector provider enum and settings

Introduce the OKTA value to the connector_provider enum (Go const,
ConnectorProviders slice, IsValid switch, GraphQL @goEnum, and the
Postgres ALTER TYPE migration) and an OktaConnectorSettings struct
holding the customer's Okta org domain.

Okta is a per-tenant IdP with no central API gateway, so the
connector is keyed on the org domain rather than a shared host. The
oktaDomain field on CreateAPIKeyConnectorInput lets the API-key flow
carry it.

Signed-off-by: Aurélien Sibiril <81782+aureliensibiril@users.noreply.github.com>

pkg/server/api/console/v1/graphql/connector.graphql

@@ -60,6 +60,7 @@ enum ConnectorProvider
         )
     CURSOR @goEnum(value: "go.probo.inc/probo/pkg/coredata.ConnectorProviderCursor")
     DATADOG @goEnum(value: "go.probo.inc/probo/pkg/coredata.ConnectorProviderDatadog")
+    OKTA @goEnum(value: "go.probo.inc/probo/pkg/coredata.ConnectorProviderOkta")
 }
 
 type ConnectorProviderInfo {
@@ -135,6 +136,7 @@ input CreateAPIKeyConnectorInput {
     metabaseInstanceUrl: String
     posthogRegion: String
     posthogInstanceUrl: String
+    oktaDomain: String
 }
 
 type CreateAPIKeyConnectorPayload {