aws-xray-sdk-fetch: move tsd from dependencies to devDependencies

### Summary

`sdk_contrib/fetch/package.json` declares [`tsd`](https://github.com/SamVerschueren/tsd) as a runtime `dependency`, but `tsd` is only used to run TypeScript declaration tests via the `test` and `test-d` scripts. It is never `require()`d by any file in `lib/`.

### Impact

`tsd@0.28.x` transitively pulls in `@tsd/typescript` (~52 MB) plus a top-level hoisted `typescript` (~24 MB). Every consumer who runs `npm install --production` or `yarn install --production` ships ~80 MB of unused TypeScript compiler binaries with their application.

For AWS Lambda users specifically, this is more than an inconvenience — it can push deployments past Lambda's 250 MB unzipped layer/function limit. We hit this directly.

After installing only `aws-xray-sdk-fetch@3.12.0` and its declared `dependencies`:

```
node_modules/tsd 1.3 MB
node_modules/@tsd 55 MB (← @tsd/typescript)
node_modules/typescript 24 MB (← hoisted via tsd → typescript)
```

### Verification that `tsd` is unused at runtime

`lib/fetch_p.js` only requires:

```js
const AWSXRay = require('aws-xray-sdk-core');
require('./subsegment_fetch');
```

`grep -r "tsd" lib/` returns nothing.

### Proposed fix

Move `tsd` from `dependencies` to `devDependencies` in `sdk_contrib/fetch/package.json`:

```diff
- "dependencies": {
- "tsd": "^0.28.1"
- }
+ "devDependencies": {
+ ...,
+ "tsd": "^0.28.1"
+ }
```

`tsd` is invoked only via `npm test` and `npm run test-d`, both of which run in dev contexts where devDependencies are present.

Happy to open a PR if useful.

Open sdk_contrib/fetch/package.json and inspect the dependency sections and the test and test-d scripts. Move tsd to the development-only section, then run npm test and npm run test-d and verify a production install no longer includes tsd or its TypeScript tooling.

Written by the indexing model from the issue text.