Add #[abi_name(name = "foo")] attribute to rename ABI items.

Add `#[abi_name(name = "foo")]` attribute to rename enum and struct ABI
items.

Here is example of how it can be used:

```
contract;

struct MyStruct {}

enum MyEnum {
  A: ()
}

abi MyAbi {
    fn my_struct() -> MyStruct;
    fn my_enum() -> MyEnum;
}

impl MyAbi for Contract {
  fn my_struct() -> MyStruct { MyStruct{} }
  fn my_enum() -> MyEnum { MyEnum::A }
}
```

Closes #5955.

Author: tritao

docs/book/src/reference/attributes.md

@@ -4,6 +4,7 @@ Attributes are a form of metadata that can additionally instruct Sway compiler o
 
 Below is the list of attributes supported by the Sway compiler, ordered alphabetically:
 
+- [ABI Name](#abi-name)
 - [Allow](#allow)
 - [Cfg](#cfg)
 - [Deprecated](#deprecated)
@@ -15,6 +16,62 @@ Below is the list of attributes supported by the Sway compiler, ordered alphabet
 - [Storage](#payable)
 - [Test](#test)
 
+## ABI Name
+
+The `#[abi_name]` attribute allows to specify the ABI name for an item.
+This means that when an ABI JSON file is generated, the name that is output is the one specified
+by the attribute. This can be useful to allow renaming items, while allowing for keeping backwards
+compatibility at the contract ABI level.
+
+> **Note**: At the moment, only enum and struct types support the attribute.
+
+In the example that follows, we originally had `MyStruct` and `MyEnum` types, which we, later on, renamed to "MyStruct" and "MyEnum" in code. To keep the backward compatibility of the ABI, we annotate the types with the `#[abi_name]` attribute and give them the original names:
+
+```sway
+contract;
+
+#[abi_name(name = "MyStruct")]
+struct RenamedMyStruct {}
+
+#[abi_name(name = "MyEnum")]
+enum RenamedMyEnum {
+  A: ()
+}
+
+abi MyAbi {
+    fn my_struct() -> RenamedMyStruct;
+    fn my_enum() -> RenamedMyEnum;
+}
+
+impl MyAbi for Contract {
+  fn my_struct() -> RenamedMyStruct { RenamedMyStruct{} }
+  fn my_enum() -> RenamedMyEnum { RenamedMyEnum::A }
+}
+```
+
+This generates the following JSON ABI:
+
+```json
+{
+  "concreteTypes": [
+    {
+      "concreteTypeId": "215af2bca9e1aa8fec647dab22a0cd36c63ce5ed051a132d51323807e28c0d67",
+      "metadataTypeId": 1,
+      "type": "enum MyEnum"
+    },
+    {
+      "concreteTypeId": "d31db280ac133d726851d8003bd2f06ec2d3fc76a46f1007d13914088fbd0791",
+      "type": "struct MyStruct"
+    }
+  ],
+  ...
+}
+```
+
+We get the same JSON ABI output both before and after renaming the types, due to attributing them with
+`#[abi_name(name = ...)]`, which forces them to be generated with their previous Sway names.
+This means consumers of this contract will still get the original names, keeping compatibility at the ABI level.
+
 ## Allow
 
 The `#[allow(...)]` attribute disables compiler checks so that certain warnings will go unreported. The following warnings can be disabled:

forc-pkg/src/pkg.rs

@@ -1791,6 +1791,7 @@ pub fn compile(
                         panic_occurrences: &asm.panic_occurrences,
                         abi_with_callpaths: true,
                         type_ids_to_full_type_str: HashMap::<String, String>::new(),
+                        unique_names: HashSet::<String>::new(),
                     },
                     engines,
                     if experimental.new_encoding {

sway-ast/src/attribute.rs

@@ -52,6 +52,10 @@ pub const ERROR_TYPE_ATTRIBUTE_NAME: &str = "error_type";
 pub const ERROR_ATTRIBUTE_NAME: &str = "error";
 pub const ERROR_M_ARG_NAME: &str = "m";
 
+// Abi names.
+pub const ABI_NAME_ATTRIBUTE_NAME: &str = "abi_name";
+pub const ABI_NAME_NAME_ARG_NAME: &str = "name";
+
 pub const KNOWN_ATTRIBUTE_NAMES: &[&str] = &[
     STORAGE_ATTRIBUTE_NAME,
     DOC_COMMENT_ATTRIBUTE_NAME,
@@ -62,6 +66,7 @@ pub const KNOWN_ATTRIBUTE_NAMES: &[&str] = &[
     CFG_ATTRIBUTE_NAME,
     DEPRECATED_ATTRIBUTE_NAME,
     FALLBACK_ATTRIBUTE_NAME,
+    ABI_NAME_ATTRIBUTE_NAME,
 ];
 
 /// An attribute declaration. Attribute declaration