Consider add a new linter rule set for brownfield services

[pshao25]

Current behavior is we have only one rule set "@azure-tools/typespec-azure-rulesets/resource-manager" to force users follow the best practices. However, lots of brownfield RP APIs predate ARM guidance, or were granted exceptions in review, and those APIs will need to suppress this rule. To give an example, our customer is a proxy resource and doesn't have a delete operation. According to their document, encryption scope cannot be deleted but can be disabled. We might consider having a linter rule set specially for brownfield services which doesn't have such strict rules as "@azure-tools/typespec-azure-rulesets/resource-manager" does.