Make CacheLocation an enum

[jodonnell]

I'm submitting a...

[ ] Regression (a behavior that used to work and stopped working in a new release)
[ ] Bug report  
[ ] Performance issue
[x] Feature request
[ ] Documentation issue or request
[ ] Other... Please describe:

Browser:

• Chrome version XX
• Firefox version XX
• IE version XX
• Edge version XX
• Safari version XX

Library version

Library version: X.Y.Z

Current behavior

when setting cacheLocation with typescript the user becomes victim to type widening so something like this is required

  const config = {
    auth: {
      authority: (process.env.REACT_APP_AUTHORITY as string),
      clientId: (process.env.REACT_APP_AAD_APP_CLIENT_ID as string),
    },
    cache: {
      cacheLocation: ('localStorage' as CacheLocation),
      storeAuthStateInCookie: true,
    },
  };

Expected behavior

I think it would be a bit clearer if CacheLocation was an enum so that may be used directly without type widening

enum CacheStorage {
  LocalStorage = "localStorage",
  SessionStorage = "sessionStorage"
}
........
  const config = {
    auth: {
      authority: (process.env.REACT_APP_AUTHORITY as string),
      clientId: (process.env.REACT_APP_AAD_APP_CLIENT_ID as string),
    },
    cache: {
      cacheLocation: CacheStorage.LocalStorage,
      storeAuthStateInCookie: true,
    },
  };

See discussion here: syncweek-react-aad/react-aad#95 (comment)

Minimal reproduction of the problem with instructions

[sameerag]

Sure. @jodonnell Are you open to submitting a PR for the same?

[jodonnell]

#851

i was only sort of able to test this in context so i'm not 100% confident it works like i want it to :'(

[sameerag]

Merged this to dev, @jodonnell Please let us know if you need anything else in this context.