Add a linter enforcing numeric durations

### Clear and concise description of the problem

`duration` defaults to a string on the wire, but Azure prohibits ISO8601 durations outside of recurrences. We should have a linting rule that validates anyone using `duration` is also specifying `@encode(DurationKnownEncoding.seconds)` for any of the numeric duration types.

### Checklist

- [x] Follow our [Code of Conduct](https://github.com/azure/typespec-azure/blob/main/CODE_OF_CONDUCT.md)
- [x] Check that this issue is about the Azure libraries for typespec. For feature request in the typespec language or core libraries file it in the [TypeSpec repo](https://github.com/Microsoft/TypeSpec/issues/new/choose)
- [x] Read the [docs](https://azure.github.io/typespec-azure/).
- [x] Check that there isn't already an issue that request the same feature to avoid creating a duplicate.

No implementation files or tests are named in the issue. Start by locating the existing lint-rule entry points and their tests, then trace how duration uses and encoding decorators are represented. Done means numeric duration types using duration are linted unless they specify @encode(DurationKnownEncoding.seconds), with coverage for the relevant cases.

Written by the indexing model from the issue text.