Reduce process count by implementing muxer validation ourselves

# Context
We validate each runtime version upon install using `dotnet --list-runtimes`. But the only validation that the muxer does:

(for runtimes)

a. checks that .deps.json exists in /shared/,
b. as well as valid semver as the folder version
https://github.com/dotnet/runtime/blob/cecdae61d396bf390ff6e2e5378dc2f804fb8606/src/native/corehost/fxr/framework_info.cpp#L79

c. blocks any runtime folder based on the version directory name, to respect 'DOTNET_DISABLE_RUNTIME_VERSIONS' https://github.com/dotnet/runtime/blob/cecdae61d396bf390ff6e2e5378dc2f804fb8606/src/native/corehost/fxr/framework_info.cpp#L98
d. the architecture is a match

(for sdks)

a. that the dotnet.dll exists:
https://github.com/dotnet/runtime/blob/b79d4c48d3eec6dcf21a0d71eb2aabda7cdaa95d/src/native/corehost/fxr/sdk_info.cpp#L64
b. that it is a valid semver folder
c. global.json/cwd influenced in terms of what dotnet root it looks into https://github.com/dotnet/runtime/blob/b79d4c48d3eec6dcf21a0d71eb2aabda7cdaa95d/src/native/corehost/fxr/sdk_resolver.cpp#L77-L100

# Suggestion

The SDK lookup with global.json / CWD is harder to mirror and more likely to break / change, so I would suggest starting with just the runtimes.

# Wins

Instead of spawning a process and calling this to validate, our logic mostly does this already today, we can incorporate the few caveats.
This is less future-proofed if the muxer changes, but it rarely does and this is worth a perf win, reducing at least:

Startup:
1 process when acquire is called to validate the runtime, OR 1 process if acquire is not needed but we validate the existing install before returning it. (Essentially true every time)
Up to 5 processes per find-path call when it validates the path, depending on if there are 5 unique dotnet installs (unlikely - usually it is only 1 to 2.)

Post Startup:
N processes for N (total installed) runtimes during the automatic updater when it validates each runtime

In the future:
N processes for N runtimes during findPath call if we ever re-add local runtime to the path search, which we can now do