Use central package management for source projects

[lbussell]

Package versions under src were declared independently in each project, which made shared dependency updates easy to miss.

This PR enables NuGet central package management for all source projects through src/Directory.Packages.props.

• Centralizes every direct package version under src
• Updates the shared Microsoft.Extensions and System package versions from 10.0.10 to 10.0.11
• Keeps packages on independent release tracks at their existing versions
• Adds package source mappings for the public and engineering feeds
• Copies the central package file into the Linux and Windows ImageBuilder container restore layers

[mthalman]

Wouldn't CPM be the more appropriate solution for this?

[lbussell]

Wouldn't CPM be the more appropriate solution for this?

1. Central package version management was historically blocked by Add a buildContext option to Platform in Manifest #1592. Since we wanted to have a Directory.Packages.props at the root of the repo, the Dockerfile in src/ couldn't resolve it. We could probably revisit it though, since all C# files and projects were recently rearranged to be under the src/ directory.
2. This orthogonal to CPM. We would still have this property even if we adopted CPM. I'm trying to force all the Microsoft.Extensions.* and System.* packages to version together.

[lbussell]

Added CPM.