Add frozen_string_literal magic comment

[yawboakye]

Adopting the almost ubiquitous frozen_string_literal magic comment to ensure efficient string storage. I observed that we don't mutate any of the string literals after we have declared them which implies that, in essence, they're already treated as frozen values. Invoking the magic comment is only a clue to the Ruby VM to optimize the strings.

Closes #1407.

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Test Plan:

All existing tests pass.

Reviewer Checklist:

• Maintains compliance with JSON:API
• Adequate test coverage exists to prevent regressions