Adds new methods to produce a SignUrlOption that results in generated
signed URLs using virtual-hosted-style URLs (i.e.
mybucket.storage.googleapis.com instead of
storage.googleapis.com/mybucket).

One option allows specifying the virtual hostname explicitly (for the
case where someone might have a custom subdomain, with a bucket of the
same name, CNAME'd to c.storage.googleapis.com), while the other will
implicitly construct the hostname using the bucket from the passed-in
BlobInfo.

Addresses part of https://issuetracker.google.com/issues/130190655.

This allows creating a signed URL in path-style (bucket in the path
component of the URI instead of in the hostname, i.e. virtual
hosted-style). Note that this was already the default behavior, but
adding this method allows us to more easily switch the default behavior
in the future; virtual hosted-style is considered best practice.

Per code review comments.

The Signature V2 doesn't require us to sign the "host" header, but the
Signature V4 process requires it. We need to ensure we're populating and
signing the same host header value as is used in the request URL.

I discovered a bug with one of my previous commits, where I was
incorrectly determining whether to use Signature Version 2. This commit
makes two new (private) methods that determine the correct default
option to use, taking default values into account.