Enable third-party Bicep linters (extensions)

**Is your feature request related to a problem? Please describe.**
Our team would like to prevent certain kinds of Bicep constructs to be used, that are not suitable for wide-use [linters](https://docs.microsoft.com/en-us/azure/azure-resource-manager/bicep/bicep-config-linter).

**Describe the solution you'd like**
Let Bicep dynamically load linter plugins from a local folder as a param.

The issue names no implementation files, tests, or entry points. Start by locating Bicep's linter configuration and extension-loading entry points, then determine how a local folder parameter should be represented and validated. Done means third-party linters load dynamically from that folder and are exercised by coverage for valid and invalid configurations.

Written by the indexing model from the issue text.