libcamera: value_node: Add mutable children accessors

Add two at() functions that return mutable pointer to child nodes, based
on an index for lists and a key for dictionaries.

The API differs from const children accessors that use operator[] and
return a reference in order to allow better error handling: while the
const accessors return a reference to a global instance of an empty
ValueNode when the requested child doesn't exist, a mutable accessor
can't do the same without allowing modifying the empty global instance.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Barnabás Pőcze <barnabas.pocze@ideasonboard.com>
Reviewed-by: Isaac Scott <isaac.scott@ideasonboard.com>

include/libcamera/internal/value_node.h

@@ -230,9 +230,11 @@ public:
 	ConstDictAdapter asDict() const { return ConstDictAdapter{ list_ }; }
 	ConstListAdapter asList() const { return ConstListAdapter{ list_ }; }
 
+	ValueNode *at(std::size_t index);
 	const ValueNode &operator[](std::size_t index) const;
 
 	bool contains(std::string_view key) const;
+	ValueNode *at(std::string_view key);
 	const ValueNode &operator[](std::string_view key) const;
 
 	ValueNode *add(std::unique_ptr<ValueNode> &&child);