Marshal unmarshal by path

[alovak]

This PR introduces path-based operations for accessing and manipulating fields and subfields in ISO 8583 messages using dot notation (e.g., "3.2.1" or "11.1").

New interfaces:

• field.PathMarshaler - marshal field values by path
• field.PathUnmarshaler - unmarshal field values by path
• field.PathUnsetter - unset fields by path

Changes:

• Added MarshalPath(path string, value any) to Message and Composite - sets field values using path notation
• Added UnmarshalPath(path string, value any) to Message and Composite - retrieves field values using path notation
• Added UnsetPath(idPaths ...string) to Message and Composite - unsets fields using path notation (replaces deprecated
  UnsetFields/UnsetSubfields)
• Fixed concurrency issue in UnsetPath methods by ensuring field operations stay within mutex locks
• Deprecated Message.UnsetFields() and Composite.UnsetSubfields() in favor of UnsetPath()

Example:

message := iso8583.NewMessage(spec)

message.MarshalPath("3.1", "12")      // Set subfield 1 of field 3

var val string
message.UnmarshalPath("3.1", &val)    // Get subfield 1 of field 3

message.UnsetPath("3.1", "4")         // Unset multiple fields

solves #383