[confmap] use DeepEqual when using confmap.enableMergeAppendOption (#12952)

<!--Ex. Fixing a bug - Describe the bug and how this fixes the issue.
Ex. Adding a feature - Explain what this achieves.-->
#### Description

`Equal(..)` panics when we're comparing maps. Use `reflect.DeepEqual`
instead. It behaves same way as `Equal` for types such as `int`,
`string`, `float` etc, but compares individual elements for maps as
well.

<!-- Issue number if applicable -->
#### Link to tracking issue
Fixes
#12932

<!--Describe what testing was performed and which tests were added.-->
#### Testing

Added a scenario

<!--Describe the documentation added.-->
#### Documentation

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Author: VihasMakwana

.chloggen/fix-panic-enableMergeAppendOption.yaml

@@ -0,0 +1,25 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: bug_fix
+
+# The name of the component, or a single word describing the area of concern, (e.g. otlpreceiver)
+component: confmap
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Use reflect.DeepEqual to avoid panic when confmap.enableMergeAppendOption feature gate is enabled.
+
+# One or more tracking issues or pull requests related to the change
+issues: [12932]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext:
+
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: []

confmap/merge.go

@@ -63,7 +63,7 @@ func mergeSlice(src, dest reflect.Value) any {
 
 func isPresent(slice reflect.Value, val reflect.Value) bool {
 	for i := 0; i < slice.Len(); i++ {
-		if slice.Index(i).Equal(val) {
+		if reflect.DeepEqual(val.Interface(), slice.Index(i).Interface()) {
 			return true
 		}
 	}

confmap/testdata/merge-append-scenarios.yaml

@@ -669,3 +669,30 @@
                 receivers: [nop]
                 processors: [attributes/example, nop2]
                 exporters: [nop]
+- name: merge-mode-map
+  configs:
+    -
+        processors:
+            resource:
+                attributes:
+                - key: deployment.region
+                  value: "nl"
+                  action: upsert
+    -
+        processors:
+            resource:
+                attributes:
+                - key: app
+                  value: "foo"
+                  action: upsert
+
+  expected: 
+    processors:
+        resource:
+            attributes:
+            - key: deployment.region
+              value: "nl"
+              action: upsert
+            - key: app
+              value: "foo"
+              action: upsert