Support for alternate credentials option

[sirknightj]

Issue #, if available:

• Why webrtc ios example contain authentication? #84
• the Cognito authentication for this sdk #61
• Does this work for unauth users? #41

Description of changes:

• Added a way for developers to skip the Cognito provisioning step when using the app. This reduces the onboarding time by a significant amount and makes it easier to switch AWS accounts during app testing.
• Added warnings in the logs for when this method is used
• Added instructions in the README for how to set it up
• I renamed AWSCredentials variable to awsCreds due to a namespace conflict.

Testing
Verified with the JS viewer the following credential methods, and they all successfully establish the RTCPeerConnection.

• Cognito (recommended path)
• AccessKey+SecreteKey
• AccessKey+SecretKey+SessionToken

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details on the usage of iam creds