@aws-crypto/client-browser Buffer usage in raw2der causing ReferenceError when decrypting

### Problem:
When using @aws-crypto/client-browser version 3.1.1, calling `decrypt()` will result in a `ReferenceError`. `decrypt` calls `raw2der`, which includes a reference to global `Buffer` type. The `buffer` npm package is not listed as a dependency in the package.json.

### Solution:
Either decouple the browser and node `ecdsa_signature.ts` helpers and use a `Uint8Array` instance instead of `Buffer` or add the [buffer](https://www.npmjs.com/package/buffer) dependency to the package.

Start with the browser package's ecdsa_signature.ts and package.json, then trace decrypt() into raw2der to confirm where the global Buffer reference is used. The fix is complete when browser decryption no longer raises ReferenceError, using either the browser-safe helper approach or the declared buffer dependency described in the issue.

索引モデルが issue の本文から書いたものです。