Support multiple CMake targets as outputs

[baszalmstra]

A single cmake project can build a library, viewers, headers, examples, tests, etc. All of these are represented as different "targets".

When we build a project, the cmake build backend installs the default (ALL_BUILD) target, which installs all targets together. However, this might not be desired. Instead, it might be desirable to produce independent conda packages.

I propose that we provide some control to the user to allow mapping cmake targets to conda package outputs:

[[tool.pixi-build-cmake.outputs]]
# The name of the conda package
name = "library"             

# The CMake target name. We will invoke `install/<cmake_target_name>` to build the package. 
# Defaults to `name` if not specified
cmake_target_name = "my_library"

[[tool.pixi-build-cmake.outputs]]
# The name of the conda package
name = "viewer"             

# Record a dependency on another cmake target. This will be reflected as a conda dependency.
# Any name mentioned here must be another output.
dependencies = ["library"]

If the outputs array is empty the default target will be packaged.