Enable building multiple projects at the command line

### Is your feature request related to a problem? Please describe.

Today when building via the dotnet CLI, only one _thing_ is built. That _thing_ may be

* a single project
* a solution file (sln, slnx, slnf)
* a C# file-based app

However, experiences like the DevKit solutionless workspace and the Aspire polyglot effort highlight the friction this introduces in our toolchain: there is no easy, built-in way to build multiple projects without a solution file. If we had such a mechanism, it would be easier for these partners to enable a nicer UX for their users, and delay the requirement of learning about solution files for new .NET users, reducing the 'slope' of the 'required concept count over time' graph.

### Describe the solution you'd like

There should be a built-in, supported way to build multiple projects easily from the command line. This support should work for all of the current commands that logically work for a solution (this list may not be exhaustive):

* build
* clean
* test
* restore
* publish
* pack

Other gestures related to these commands, like `--property`, `--framework`, and so on should apply to all projects in the set of buildable things, to maintain existing behaviors as much as possible in this new mechanism with the existing solution/project support.

UPDATE (tmat):
To support polyglot Aspire we need `build` to be able to accept not just multiple projects but potentially a mix of multiple projects and files (`--project` and `--file` options). E.g. some resource projects in Aspire might be single-file apps others might be first class projects. They all need to be built at the same time as roots of the project graph.