collab: Add endpoint for initiating a billing subscription (#15452)

This PR adds a new `POST /billing/subscriptions` endpoint that can be
used to initiate a billing subscription.

The endpoint will use the provided `github_user_id` to look up a user,
generate a Stripe Checkout session, and then return the URL.

The caller would then redirect the user to the URL to initiate the
checkout flow.

Here's an example of how to call it:

```sh
curl -X POST "http://localhost:8080/billing/subscriptions" \
     -H "Authorization: <ADMIN_TOKEN>" \
     -H "Content-Type: application/json" \
     -d '{"github_user_id": 12345}'
```

Release Notes:

- N/A

Cargo.lock

@@ -834,6 +834,26 @@ dependencies = [
  "syn 2.0.59",
 ]
 
+[[package]]
+name = "async-stripe"
+version = "0.37.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e2f14b5943a52cf051bbbbb68538e93a69d1e291934174121e769f4b181113f5"
+dependencies = [
+ "futures-util",
+ "http-types",
+ "hyper",
+ "hyper-rustls",
+ "serde",
+ "serde_json",
+ "serde_path_to_error",
+ "serde_qs 0.10.1",
+ "smart-default",
+ "smol_str",
+ "thiserror",
+ "tokio",
+]
+
 [[package]]
 name = "async-tar"
 version = "0.4.2"
@@ -1462,6 +1482,12 @@ version = "0.1.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "349a06037c7bf932dd7e7d1f653678b2038b9ad46a74102f1fc7bd7872678cce"
 
+[[package]]
+name = "base64"
+version = "0.13.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9e1b586273c5702936fe7b7d6896644d8be71e6314cfe09d3167c95f712589e8"
+
 [[package]]
 name = "base64"
 version = "0.21.7"
@@ -2425,6 +2451,7 @@ dependencies = [
  "anthropic",
  "anyhow",
  "assistant",
+ "async-stripe",
  "async-trait",
  "async-tungstenite",
  "audio",
@@ -5254,6 +5281,27 @@ version = "0.3.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "add0ab9360ddbd88cfeb3bd9574a1d85cfdfa14db10b3e21d3700dbc4328758f"
 
+[[package]]
+name = "http-types"
+version = "2.12.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6e9b187a72d63adbfba487f48095306ac823049cb504ee195541e91c7775f5ad"
+dependencies = [
+ "anyhow",
+ "async-channel 1.9.0",
+ "base64 0.13.1",
+ "futures-lite 1.13.0",
+ "http 0.2.9",
+ "infer",
+ "pin-project-lite",
+ "rand 0.7.3",
+ "serde",
+ "serde_json",
+ "serde_qs 0.8.5",
+ "serde_urlencoded",
+ "url",
+]
+
 [[package]]
 name = "http_client"
 version = "0.1.0"
@@ -5512,6 +5560,12 @@ version = "2.0.5"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "b248f5224d1d606005e02c97f5aa4e88eeb230488bcc03bc9ca4d7991399f2b5"
 
+[[package]]
+name = "infer"
+version = "0.2.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "64e9829a50b42bb782c1df523f78d332fe371b10c661e78b7a3c34b0198e9fac"
+
 [[package]]
 name = "inherent"
 version = "1.0.10"
@@ -9564,6 +9618,28 @@ dependencies = [
  "serde",
 ]
 
+[[package]]
+name = "serde_qs"
+version = "0.8.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "c7715380eec75f029a4ef7de39a9200e0a63823176b759d055b613f5a87df6a6"
+dependencies = [
+ "percent-encoding",
+ "serde",
+ "thiserror",
+]
+
+[[package]]
+name = "serde_qs"
+version = "0.10.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "8cac3f1e2ca2fe333923a1ae72caca910b98ed0630bb35ef6f8c8517d6e81afa"
+dependencies = [
+ "percent-encoding",
+ "serde",
+ "thiserror",
+]
+
 [[package]]
 name = "serde_repr"
 version = "0.1.16"
@@ -9880,6 +9956,17 @@ dependencies = [
  "serde",
 ]
 
+[[package]]
+name = "smart-default"
+version = "0.6.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "133659a15339456eeeb07572eb02a91c91e9815e9cbc89566944d2c8d3efdbf6"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn 1.0.109",
+]
+
 [[package]]
 name = "smol"
 version = "1.3.0"
@@ -9897,6 +9984,15 @@ dependencies = [
  "futures-lite 1.13.0",
 ]
 
+[[package]]
+name = "smol_str"
+version = "0.1.24"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "fad6c857cbab2627dcf01ec85a623ca4e7dcb5691cbaa3d7fb7653671f0d09c9"
+dependencies = [
+ "serde",
+]
+
 [[package]]
 name = "snippet"
 version = "0.1.0"