[java-storage] Allow to create presigned URLs using HMAC keys

**Is your feature request related to a problem? Please describe.**
Currently `com.google.cloud.storage.StorageImpl` class allows to create a presigned URL using RSA algorithm. One can pass their own `ServiceAccountSigner` implementation (via `StorageOptions.setCredentials()`) but the algorithm name literal is deeply hard-coded as `GOOG4-RSA-SHA256` (also in `com.google.cloud.storage.SignatureInfo`). It's not possible to efficiently reuse current code to sign URL via HMAC keys (https://cloud.google.com/storage/docs/authentication/signatures#signing-process).

**Describe the solution you'd like**
Allow to pass an algorithm via `StorageOptions`.

**Describe alternatives you've considered**
The only solution I see is to copy/fork all the code. Reflection seems to be too fragile.

首先阅读 com.google.cloud.storage.StorageImpl 和 com.google.cloud.storage.SignatureInfo，重点关注选择 GOOG4-RSA-SHA256 算法的位置，以及 StorageOptions 如何提供凭据。当调用方可以通过 StorageOptions 配置签名算法，并使用 HMAC 密钥创建预签名 URL，而无需复制或 fork 现有代码时，这项工作就完成了。

由索引模型根据 Issue 内容生成。