Add ability to pass a custom crypto instance

[stewones]

This is useful for testing PKCE in local environments.

example:

    class CustomCrypto {
      generateRandom(size: number): string {
        // implement with your own crypto method. eg: using the sha256 lib
        return 'random_' + size;
      }
      deriveChallenge(code: string) {
        // implement with your own crypto method. eg: using the sha256 lib
        return Promise.resolve('challenge_' + code);
      }
    }

  const authService = new AuthService(browser, storage, requestor, new CustomCrypto());

also added a test for this new behavior, check the auth-service.spec.ts

fixes #73

[mraible]

@wi3land Can you please review this? It seems to change a lot of code that's not related to the feature.

[stewones]

@wi3land Can you please review this? It seems to change a lot of code that's not related to the feature.

I'm proposing to ignore build files from git.

[stewones]

yo! any updates?
so to make it clear, I'm just proposing to:

1. remove the final build files from git. this is a terrible idea, but I understand if the author disagrees and I can happily revert to its original setup. just let me know.
   61bab14

2. allow the AuthService to accept a custom crypto implementation so it's easier for us to test PKCE challenges in non-secure environments
   3f544b6

[jsonhero]

Please add this!