Use instance discovery for trusted hosts and combine authority classes

[tnorling]

This PR does several things in regards to Authorities:

• Removes the hardcoded trusted host list for AAD hosts and replaces it with a network call to discover hosts known to MS. This is done the first time resolveEndpointsAsync is called.
• Combines B2C and AAD authority classes into a single generic class

Developers can opt out of the network call in one of 3 ways:

1. Set validateAuthority: false
2. Provide all trusted authorities they intend to use in knownAuthorities config option
3. Provide instance metadata via authorityMetadata config option

Note: This does not include preferred_cache or preferred_network fields and does not include a scheduled refresh. These can be added in separate PRs if needed for core.

[coveralls]

Coverage decreased (-0.2%) to 76.627% when pulling 84dd6a2 on cloud-discovery into 8c86372 on dev.

[tnorling]

Per discussion: First invocation of resolveEndpointsAsync will call Discovery Endpoint if TrustedHostList is not already set.

[jmckennon]

As a general thought, do we want to include logging in PRs going forward? Probably most applicable to the new network requests in this PR. Otherwise LGTM

[DarylThayil]

Largely looks good to me, thanks for all the tests!

[pkanher617]

Can we create a utility class with static functions instead of exporting function constants?

[tnorling]

I think that's reasonable but since this PR isn't really changing any telemetry files other than to fix compilation, I think that can be taken separately.