refactor: Add constants for all roles in LTI 1.3 spec
refactor: Add constants for all roles in LTI 1.3 spec

I think it will be helpful to have a definitive list of all LTI 1.3 roles and have them set in constants so we can reference the constants instead of having multiple instances of roles as strings throughout the codebase.

atomicjolt/adhesion,atomicjolt/lti_starter_app,atomicjolt/lti_starter_app,atomicjolt/adhesion,atomicjolt/adhesion,atomicjolt/lti_starter_app,atomicjolt/lti_starter_app,atomicjolt/adhesion