Add cmake flag for iot creds

[sirknightj]

Issue #, if available:
N/A

What was changed?
Add a CMake flag that defines the pre-processor macro: IOT_CORE_ENABLE_CREDENTIALS. This flag is used in the samples to enable IoT credentials. Otherwise, static credentials (AK, SK, (ST)) are used.

Why was it changed?

• Makes it easier to build. Previously, you would need to uncomment a line in the samples to enable using IoT credentials in the samples.

How was it changed?

• If "-DIOT_CORE_ENABLE_CREDENTIALS=ON" cmake flag is present, it will add the pre-processor macro: IOT_CORE_ENABLE_CREDENTIALS. Also add a log line to indicate that it chose this path. To maintain backwards compatibility, this flag is OFF by default.

What testing was done for the changes?

1. cmake .., run kvsWebrtcClientMaster
   • IoT credentials are not used.
2. cmake .. -DIOT_CORE_ENABLE_CREDENTIALS=OFF, run kvsWebrtcClientMaster
   • IoT credentials still not used.
3. cmake .. -DIOT_CORE_ENABLE_CREDENTIALS=ON, run kvsWebrtcClientMaster
   • IoT credentials are used.

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[hassanctech]

lgtm