[FEATURE REQ][Key Vault JCA] QoL changes for `HttpUtil`.

Just as outlined by #23547, the Key Vault JCA library is not up to par with other similar data plane libraries in our Azure SDK for Java. As part of the effort to align the library with the rest, we also need to add QoL changes to the `HttpUtil` class such as:
- Avoid creating a new `HttpClient` every time an HTTP request needs to be made and instead use a longer-lived instance wherever possible.
- Stop returning `null` in methods such as `get()` or `post()` when there's an error in sending a request/ receiving a response and instead bubble up a proper exception.
- Add logic to retry on transient errors, much like what is done in Azure Core's `RetryPolicy`.