Change production code paths for Composer to use the new transport
agnostic interface.

Required a few setup steps to make tests pass, which will be needed for
other integrations too:
* Add initialization of tc to NewClient
* Add new useGRPC flag to remove switches on tc in Reader/Writer

Also a few cleanups:
* Use EncryptionKey from dstObject
* Use defaultGen instead of hard coded int

Also, a unit test checks that setting Generation with composer causes
a failure. Currently this is enforced by applyConds. I added
a separate validation for this instead, but I could do a pass-
through of the Generation through the interface -- it's just
confusing looking because setting Generation on a new object
is never valid.

This takes out a few lines of boilerplate that will be
present in all top-level methods.

Migrate Copier to use the new transport-agnostic interface.
I found one additional piece of data that needs to be passed back
through the interface in the process (object size).

I removed one integration test. This test is old and currently
calls rewriteObject in a way that an external library user could
not accomplish. I don't think this test adds any value.

* chore(storage): migrate GetServiceAccount

* migrate ListBuckets

* migrate CreateBucket and refactor interface method

* update tests and BucketIterator with retryConfig

* Revert "update tests and BucketIterator with retryConfig"

This reverts commit b7662b4.

* remove unused field and internal method upon migration

* chore(storage): migrate IAM operations

* remove unused field

Migrate ACL ops to the new transport agnostic interface. No
interface changes were necessary for this one and tests pass.

Migrates the Notification APIs to the transport agnostic surface. Integration tests passed and added a check to ensure that AddNotification returns a Notification with a non-empty ID.

* test: update continuous.sh for storage-refactor

* add todo to revert change

Co-authored-by: Chris Cotter <cjcotter@google.com>
Co-authored-by: cojenco <cathyo@google.com>

* chore(storage): migrate GetBucket and DeleteBucket

* migrate remaining bucket methods

* fix merge conflict

Co-authored-by: Chris Cotter <cjcotter@google.com>

Migrate Object metadata operations to transport-agnostic interface. Integration tests pass locally
- GetObject
- UpdateObject
- DeleteObject

Migrate Writer to use the transport-agnostic interface. A few
minor fixes:

* Pass in retry, idempotency and user project via settings rather
than determining below interface.
* Use default generation since this can't be sent for this call.

Migrate the Reader to use the new transport-agnostic interface.
A few minor fixes were required:
* Move validations above interface
* Fix up settings for call
* Add param for sending with accept-encoding: gzip (HTTP only)
* Fix up reader resumption test