[Proposal] Introducing new compatibility level "Warning" for changes which are technically non-breaking but can cause unforeseen behavior change for client

## Introducing compatibility level "Warning"
There are certain changes diff tool reports which are technically non-breaking but can cause unforeseen behavior changes in client and should be reported differently. Cases where this is applicable:
- Default value is added/removed/changed
- Extension is added/removed
- An enum value is added