Make network reachability provider mockable

[gleue]

Description of changes:

Merging this PR will make

• initializer AWSAppSyncClient.init(appSyncConfig:reachabilityFactory:) public, and
• protocols NetworkReachabilityProvidingFactory and NetworkReachabilityProviding public

which are all internal to the framework for now.

Rationale behind changes:

By allowing framework users access to above API it will be possible to

• mock the reachability service properly for integration tests
• substitute the reachability service with a private solution if already present in a project

Test results

The proposed changes have no effect on unit or integration tests. All tests succeed w/ changes
applied.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[palpatim]

Thanks for the PR!