Add DefaultAzureCredential

[antkmsft]

Closes #82.

It is all the credential that I presented earlier, implementation is very simple, but 95% of other changes are instrumenting the code with log messages, and covering all that with tests + update to the docs (heavily borrowed from .NET SDK's identity readme)

Here is how the logs look like when you use DefaultAzureCredential: https://github.com/Azure/azure-sdk-for-cpp/blob/51df426b557f8b29c2037aba53500418c73e8d07/sdk/identity/azure-identity/test/ut/default_azure_credential_test.cpp

[gearama]

Ancient bug 80s range

[antkmsft]

This came up a couple times already, so I am posting for everyone's view - here is how the logs look like when you use DefaultAzureCredential: https://github.com/Azure/azure-sdk-for-cpp/blob/51df426b557f8b29c2037aba53500418c73e8d07/sdk/identity/azure-identity/test/ut/default_azure_credential_test.cpp

[antkmsft]

/azp run cpp - identity

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