ExtensionsMetadataGenerator: Support excluding assemblies from extension probing

Had an internal request to support excluding specific assemblies from our extension probing logic. The scenario was that they were pulling an assembly from another project into a new project, but didn't want the `FunctionsStartup` to run from the dependency.

The workaround is fairly easy -- split out the logic into a separate assembly and reference that -- but it would be nice to have the ability to specify in the project file that an assembly should be explicitly ignored.

Start by locating ExtensionsMetadataGenerator and the extension probing entry point in the host. Determine how project-file settings could identify assemblies to exclude, then verify that an excluded dependency does not run its FunctionsStartup; the issue does not name tests or files for validation.

Written by the indexing model from the issue text.