Show warning message if valueFrom is used instead of value_from in app config section (#4429)

## Changes
Show warning message if camel case field is used instead of snake case

## Why
DABs syntax uses snake case for all the configuration fields. Sometimes
users might use camelCase instead which leads to a warning. With this
change we guide users to use the correct field name.

Fixes
#4297 (comment)

## Tests
Added an acceptance test

<!-- If your PR needs to be included in the release notes for next
release,
add a separate entry in NEXT_CHANGELOG.md as part of your PR. -->

Author: andrewnester

NEXT_CHANGELOG.md

@@ -15,6 +15,7 @@
 * Added support for UC catalogs (only in direct mode) ([#4342](https://github.com/databricks/cli/pull/4342))
 * engine/direct: Fix updates to fields being ignored for database_catalogs, synced_database_tables (([#4388](https://github.com/databricks/cli/pull/4388)))
 * engine/direct: Fix migration for database_instances, database_catalog, synced_database_tables ([#4424](https://github.com/databricks/cli/pull/4424))
+* Show warning message if valueFrom is used instead of value_from in app config section ([$4429](https://github.com/databricks/cli/pull/4429))
 
 ### Dependency updates
 

acceptance/bundle/apps/value_from_warning/app/app.py

@@ -0,0 +1 @@
+print("Hello from the app!")

acceptance/bundle/apps/value_from_warning/databricks.yml

@@ -0,0 +1,12 @@
+bundle:
+  name: test-bundle
+
+resources:
+  apps:
+    myapp:
+      name: myapp
+      source_code_path: ./app
+      config:
+        env:
+          - name: MY_SECRET
+            valueFrom: secret://my_secret

acceptance/bundle/apps/value_from_warning/out.test.toml

@@ -0,0 +1,15 @@
+
+>>> [CLI] bundle validate
+Warning: Use 'value_from' instead of 'valueFrom'
+  at resources.apps.myapp.config.env[0]
+  in databricks.yml:12:13
+
+The field 'valueFrom' should be 'value_from' (snake_case). The 'valueFrom' field will be ignored.
+
+Name: test-bundle
+Target: default
+Workspace:
+  User: [USERNAME]
+  Path: /Workspace/Users/[USERNAME]/.bundle/test-bundle/default
+
+Found 1 warning

acceptance/bundle/apps/value_from_warning/output.txt

@@ -0,0 +1 @@
+trace $CLI bundle validate

acceptance/bundle/apps/value_from_warning/script

@@ -0,0 +1,5 @@
+RecordRequests = false
+
+Ignore = [
+    '.databricks',
+]

acceptance/bundle/apps/value_from_warning/test.toml

@@ -98,13 +98,27 @@ func (n normalizeOptions) normalizeStruct(typ reflect.Type, src dyn.Value, seen
 			pk := pair.Key
 			pv := pair.Value
 
-			index, ok := info.Fields[pk.MustString()]
+			fieldName := pk.MustString()
+			index, ok := info.Fields[fieldName]
 			if !ok {
 				if !pv.IsAnchor() {
+					// Special case: provide a more helpful message for "valueFrom" vs "value_from"
+					if fieldName == "valueFrom" {
+						if _, hasValueFrom := info.Fields["value_from"]; hasValueFrom {
+							diags = diags.Append(diag.Diagnostic{
+								Severity:  diag.Warning,
+								Summary:   "Use 'value_from' instead of 'valueFrom'",
+								Detail:    "The field 'valueFrom' should be 'value_from' (snake_case). The 'valueFrom' field will be ignored.",
+								Locations: pk.Locations(),
+								Paths:     []dyn.Path{path},
+							})
+							continue
+						}
+					}
+
 					diags = diags.Append(diag.Diagnostic{
-						Severity: diag.Warning,
-						Summary:  "unknown field: " + pk.MustString(),
-						// Show all locations the unknown field is defined at.
+						Severity:  diag.Warning,
+						Summary:   "unknown field: " + fieldName,
 						Locations: pk.Locations(),
 						Paths:     []dyn.Path{path},
 					})