chore: add indexmap dependency for cargo

Cargo.lock

@@ -23,6 +23,18 @@ version = "1.0.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "9555578bc9e57714c812a1f84e4fc5b4d21fcb063490c624de019f7464c91268"
 
+[[package]]
+name = "equivalent"
+version = "1.0.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "877a4ace8713b0bcf2a4e7eec82529c029f1d0619886d18145fea96c3ffe5c0f"
+
+[[package]]
+name = "hashbrown"
+version = "0.15.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "5971ac85611da7067dbfcabef3c70ebb5606018acd9e2a3903a0da507521e0d5"
+
 [[package]]
 name = "heck"
 version = "0.5.0"
@@ -33,9 +45,20 @@ checksum = "2304e00983f87ffb38b55b444b5e3b60a884b5d30c0fca7d82fe33449bbe55ea"
 name = "httpx"
 version = "0.28.1"
 dependencies = [
+ "indexmap",
  "pyo3",
 ]
 
+[[package]]
+name = "indexmap"
+version = "2.9.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "cea70ddb795996207ad57735b50c5982d8844f38ba9ee5f1aedcfb708a2aa11e"
+dependencies = [
+ "equivalent",
+ "hashbrown",
+]
+
 [[package]]
 name = "indoc"
 version = "2.0.6"

Cargo.toml

@@ -8,6 +8,7 @@ name = "httpx"
 crate-type = ["cdylib"]
 
 [dependencies]
+indexmap = "2.9.0"
 pyo3 = { version = "0.24.0", features = [
     "extension-module",
     "generate-import-lib",