Begin targets logic

[benvillalobos]

Work Done

Created two targets, ComputeContainerConfig and PublishContainer. ComputeContainerConfig is intended to gather all info, PublishContainer is meant to do the actual work of publishing (presumably we just call a task?). ComputeContainerConfig is meant to be a "point of no return," where users should no longer modify properties/items that affect the container generation process.

Will need to coordinate more with #11 to nail down what info to grab in the targets.

Notes

I left a bunch of questions inline, this pr is expected to be a work in progress for a while.

Questions

• Where should this targets file live?
• Are the names of the targets okay? cc @baronfel
• Not entirely sure if these targets should return anything. Would we ever expect an MSBuild call on a project that intends to gather info from ComputeContainerConfig?

[benvillalobos]

Big win of the day: Figuring out that this is the right answer for constructing the entrypoint in self contained apps:
<ContainerEntrypoint Condition="'$(ContainerEntrypoint)' == '' and '$(SelfContained)' == 'true'">$(ContainerWorkingDirectory)/$(TargetDir)$(AssemblyName)$(_NativeExecutableExtension)</ContainerEntrypoint>