Write more easily diff-able logs during type generation

[anthony-c-martin]

There's no straightforward way to review the logs written during type generation, because versioning and timing is logged, causing a diff in every file (example screenshot below).

We should write the logs in a structured manner - to display the following, so that the PR can be assessed for impact:

• Added/updated/removed resource types
• Differences in warnings/errors

We should then be able to generate a summary description for each PR.

[jeskew]

This was partially resolved in #921. The generator's log level will now default to warning, so only warnings, errors, and a brief header will appear in each log file. Unfortunately, that header reports the versions used of both node and the AutoRest CLI, so there's still a bit of work to do.