feat: create user scim v2 endpoint (#9132)

# Which Problems Are Solved
- Adds infrastructure code (basic implementation, error handling,
middlewares, ...) to implement the SCIM v2 interface
- Adds support for the user create SCIM v2 endpoint

# How the Problems Are Solved
- Adds support for the user create SCIM v2 endpoint under `POST
/scim/v2/{orgID}/Users`

# Additional Context

Part of #8140

internal/api/scim/middleware/scim_context_middleware.go

@@ -0,0 +1,54 @@
+package middleware
+
+import (
+	"context"
+	"net/http"
+
+	"github.com/zitadel/zitadel/internal/api/authz"
+	zhttp "github.com/zitadel/zitadel/internal/api/http/middleware"
+	smetadata "github.com/zitadel/zitadel/internal/api/scim/metadata"
+	"github.com/zitadel/zitadel/internal/query"
+	"github.com/zitadel/zitadel/internal/zerrors"
+)
+
+func ScimContextMiddleware(q *query.Queries) func(next zhttp.HandlerFuncWithError) zhttp.HandlerFuncWithError {
+	return func(next zhttp.HandlerFuncWithError) zhttp.HandlerFuncWithError {
+		return func(w http.ResponseWriter, r *http.Request) error {
+			ctx, err := initScimContext(r.Context(), q)
+			if err != nil {
+				return err
+			}
+
+			return next(w, r.WithContext(ctx))
+		}
+	}
+}
+
+func initScimContext(ctx context.Context, q *query.Queries) (context.Context, error) {
+	data := smetadata.ScimContextData{
+		ProvisioningDomain:          "",
+		ExternalIDScopedMetadataKey: smetadata.ScopedKey(smetadata.KeyExternalId),
+	}
+
+	ctx = smetadata.SetScimContextData(ctx, data)
+
+	userID := authz.GetCtxData(ctx).UserID
+	metadata, err := q.GetUserMetadataByKey(ctx, false, userID, string(smetadata.KeyProvisioningDomain), false)
+	if err != nil {
+		if zerrors.IsNotFound(err) {
+			return ctx, nil
+		}
+
+		return ctx, err
+	}
+
+	if metadata == nil {
+		return ctx, nil
+	}
+
+	data.ProvisioningDomain = string(metadata.Value)
+	if data.ProvisioningDomain != "" {
+		data.ExternalIDScopedMetadataKey = smetadata.ScopeExternalIdKey(data.ProvisioningDomain)
+	}
+	return smetadata.SetScimContextData(ctx, data), nil
+}