Add Conan packages to the `list project dependencies` API

Context

The List Project Dependencies API call has an optional attribute for package_manager. I.e. calling GET /projects/:id/dependencies?package_manager=npm will only show results for npm packages.

Conan dependency management has been supported since #225218 (closed), and Conan packages do show up in the full results if the package_manager attribute is not used, however there is still no package_manager option to show only conan results.

Proposal

Update the API to allow passing conan as a package_manager attribute, so that I can quickly filter my dependencies list on my C/C++ dependencies.

Implementation Plan

1. Add conan to the FILTER_PACKAGE_MANAGERS_VALUES variable:

   -    FILTER_PACKAGE_MANAGERS_VALUES = %w(bundler yarn npm maven composer pip).freeze
   +    FILTER_PACKAGE_MANAGERS_VALUES = %w(bundler yarn npm maven composer pip conan).freeze

2. Add a test to Security::DependencyListService to verify that the above change works as expected