[Feature]: Add support for client credentials grant

[fwaadahmad1]

WHY are these changes introduced?

• Resolves https://github.com/Shopify/develop-app-access/issues/622

• Similar PR for shopify-api-js library: [Feature]: added support client credentials in shopify-api-js shopify-app-js#2053

WHAT is this pull request doing?

• No UI/UX Changes
• Introduces Client credentials token acquisition flow.

How has this been tested?

What should the reviewers focus on?

• Existing token_exchage and oauth flows both use create_session file, see that changes do not affect the current behaviour of the library.
• ShopifyApi::Auth::Session.from now supports expiration for both online and offline tokens.

Setup Instructions

1. Setup a Spin instance with constellation partners:dev
   • Run spin up partners:dev --wait
2. clone the client-creds-tophat-ruby app
3. clone the shopify-api-ruby library and checkout to current branch.

Tophatting

Video

Instructions

1. Make sure the path in Gemfile, in client-creds-tophat-ruby app is correct. It should be a relative path to your locally cloned shopify-api-ruby library.

gem "shopify_api", path: "../../shopify-api-ruby"

2. publish the app to your spin instance by running

shopify app dev --reset

3. Install the app to your store.
4. Open Developer tools console.
5. Click the button Perform Client Credentials Request, in your app
   • This button will call an api on the ruby server that will initiate the client_credentials flow from shopify-api-ruby library and then return the response.
   • The logs for this api call can be seen in the app's console

6. Check the console for the response containing an access_token, and expires keys.

Checklist:

• My commit message follow the pattern described in here
• I have performed a self-review of my own code.
• I have added tests that prove my fix is effective or that my feature works.
• I have updated the project documentation.
• I have added a changelog line.

[lizkenyon]

thank you! ⭐

[RyanDJLee]

Had some minor comments, but looks great!