Add AbiEncode and AbiDecode for String (#6037)

## Description

`AbiEncode` and `AbiDecode` are missing for the `String` type. This is
required for inline tests being developed in
[sway-standards](FuelLabs/sway-standards#91).

## Checklist

- [ ] I have linked to any relevant issues.
- [x] I have commented my code, particularly in hard-to-understand
areas.
- [ ] I have updated the documentation where relevant (API docs, the
reference, and the Sway book).
- [ ] If my change requires substantial documentation changes, I have
[requested support from the DevRel
team](https://github.com/FuelLabs/devrel-requests/issues/new/choose)
- [x] I have added tests that prove my fix is effective or that my
feature works.
- [x] I have added (or requested a maintainer to add) the necessary
`Breaking*` or `New Feature` labels where relevant.
- [x] I have done my best to ensure that my PR adheres to [the Fuel Labs
Code Review
Standards](https://github.com/FuelLabs/rfcs/blob/master/text/code-standards/external-contributors.md).
- [x] I have requested a review from the relevant team or maintainers.

sway-lib-std/src/string.sw

@@ -279,6 +279,35 @@ impl Hash for String {
     }
 }
 
+impl AbiEncode for String {
+    fn abi_encode(self, buffer: Buffer) -> Buffer {
+        // Encode the length
+        let mut buffer = self.bytes.len().abi_encode(buffer);
+
+        // Encode each byte of the string
+        let mut i = 0;
+        while i < self.bytes.len() {
+            let item = self.bytes.get(i).unwrap();
+            buffer = item.abi_encode(buffer);
+            i += 1;
+        }
+
+        buffer
+    }
+}
+
+impl AbiDecode for String {
+    fn abi_decode(ref mut buffer: BufferReader) -> Self {
+        // Get length and string data
+        let len = u64::abi_decode(buffer);
+        let data = buffer.read_bytes(len);
+        // Create string from the ptr and len as parts of a raw_slice
+        String {
+            bytes: Bytes::from(raw_slice::from_parts::<u8>(data.ptr(), len)),
+        }
+    }
+}
+
 // Tests
 
 #[test]
@@ -521,3 +550,18 @@ fn string_test_hash() {
 
     assert(sha256(string) == sha256(bytes));
 }
+
+#[test]
+fn string_test_abi_encoding() {
+    let string = String::from_ascii_str("fuel");
+
+    let buffer = Buffer::new();
+    let encoded_string = string.abi_encode(buffer);
+
+    let encoded_raw_slice = encoded_string.as_raw_slice();
+    let mut buffer_reader = BufferReader::from_parts(encoded_raw_slice.ptr(), encoded_raw_slice.number_of_bytes());
+
+    let decoded_string = String::abi_decode(buffer_reader);
+
+    assert(string == decoded_string);
+}