feat: add preamble for dart generated files

frb_codegen/src/binary/commands.rs

@@ -88,6 +88,10 @@ pub(crate) struct GenerateCommandArgsPrimary {
     #[arg(long)]
     pub dart_format_line_length: Option<u32>,
 
+    /// Raw header of output generated Dart code, pasted as-it-is.
+    #[arg(long)]
+    pub dart_preamble: Option<String>,
+
     /// The generated Dart enums will not have their variant names camelCased.
     #[arg(long)]
     pub no_dart_enums_style: bool,

frb_codegen/src/binary/commands_parser.rs

@@ -52,6 +52,7 @@ fn compute_codegen_config_from_naive_command_args(
         rust_output: args.rust_output,
         dart_entrypoint_class_name: args.dart_entrypoint_class_name,
         dart_format_line_length: args.dart_format_line_length,
+        dart_preamble: args.dart_preamble,
         dart_enums_style: Some(!args.no_dart_enums_style),
         add_mod_to_lib: Some(!args.no_add_mod_to_lib),
         llvm_path: args.llvm_path,

frb_codegen/src/library/codegen/config/config.rs

@@ -15,6 +15,7 @@ pub struct Config {
     pub rust_output: Option<String>,
     pub dart_entrypoint_class_name: Option<String>,
     pub dart_format_line_length: Option<u32>,
+    pub dart_preamble: Option<String>,
     pub dart_enums_style: Option<bool>,
     pub add_mod_to_lib: Option<bool>,
     pub llvm_path: Option<Vec<String>>,

frb_codegen/src/library/codegen/config/internal_config_parser.rs

@@ -119,6 +119,7 @@ impl InternalConfig {
                     dart_entrypoint_class_name: dart_output_class_name_pack
                         .entrypoint_class_name
                         .clone(),
+                    dart_preamble: config.dart_preamble.clone().unwrap_or_default(),
                 },
                 wire: GeneratorWireInternalConfig {
                     dart: GeneratorWireDartInternalConfig {

frb_codegen/src/library/codegen/generator/api_dart/internal_config.rs

@@ -7,4 +7,5 @@ pub(crate) struct GeneratorApiDartInternalConfig {
     pub dart3: bool,
     pub dart_decl_base_output_path: PathBuf,
     pub dart_entrypoint_class_name: String,
+    pub dart_preamble: String,
 }

frb_codegen/src/library/codegen/generator/wire/dart/spec_generator/misc/mod.rs

@@ -80,7 +80,9 @@ fn generate_boilerplate(
         ..
     } = &context.config.dart_output_class_name_pack;
 
+    let dart_preamble = &context.api_dart_config.dart_preamble.as_str();
     let file_top = generate_code_header()
+        + if !dart_preamble.is_empty() {"\n\n"} else {""} + dart_preamble
         + "\n\n// ignore_for_file: unused_import, unused_element, unnecessary_import, duplicate_ignore, invalid_use_of_internal_member, annotate_overrides, non_constant_identifier_names, curly_braces_in_flow_control_structures, prefer_const_literals_to_create_immutables, unused_field\n";
 
     let mut universal_imports = generate_import_dart_api_layer(