Read latest version from product-versions.json

[damonbarry]

The "aziot-version" check in aziotctl check reads the latest identity service version from https://github.com/Azure/azure-iotedge/blob/main/latest-aziot-identity-service.json. This JSON file has no ability to accomodate multiple releases (e.g., 1.4 and 1.5). However, https://github.com/Azure/azure-iotedge/blob/main/product-versions.json was recently introduced and, while it has a more complex structure, it is capable of providing information about multiple versions of the same product.

This change updates aziotctl check to read the latest identity service version from product-versions.json. Since the logic needs to contend with multiple versions, it now uses the MAJOR.MINOR version from the actual version string to match the expected version. For example, if the user has 1.5.0 installed and 1.5.2 and 1.4.10 are the latest versions, it will match on 1.5 and return 1.5.2 as the expected version. It does not differentiate between the same versions on different channels (e.g., stable vs. LTS); it assumes they are the same (in other words, it assumes product-versions.json will never legitimately show version 1.5.3 in the stable channel and 1.5.2 in the LTS channel).

[damonbarry]

@arsing @gordonwang0 I had to dust off my Rust skills for this, which were basic to begin with. I'm open to any feedback about idiomatic usage, etc.