Move JSONWebToken::HMACToken out from EE

What does this MR do?

JSONWebToken::HMACToken previously existed in ee/lib/json_web_token/hmac_token.rb but does not contain any EE specific logic and I'm planning on using it within CE. This MR does the following:

• Moves JSONWebToken::HMACToken to lib/json_web_token/hmac_token.rb
• Adds .decode()
• Adds tests (there were none)

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/8114

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [ ] Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• [ ] Conforms to the database guides
• [ ] Link to e2e tests MR added if this MR has Requires e2e tests label. See the [Test Planning Process](https://about.gitlab.com/handbook/engineering/quality/test-engineering/).
• [ ] EE specific content should be in the top level /ee folder
• [ ] For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?