storage: separate each sample into a separate file

[quartzmo]

Moved from GoogleCloudPlatform/ruby-docs-samples#481, in coordination with #5572 and #5574.

Each sample should be in a standalone file.

Question: How should the new file structure look?

Current structure:

├── Gemfile
├── Gemfile.lock
├── README.md
├── Rakefile
├── acceptance
│   ├── acls_test.rb
│   ├── buckets_test.rb
│   ├── data
│   │   └── file.txt
│   ├── files_test.rb
│   ├── helper.rb
│   ├── hmac_test.rb
│   ├── iam_test.rb
│   └── quickstart_test.rb
├── acls.rb
├── buckets.rb
├── files.rb
├── hmac.rb
├── iam.rb
└── quickstart.rb

[quartzmo]

/cc @frankyn @JesseLovelace