Add design document for Mobile runtimeconfig.json host configuration

[fanyang-mono]

Contributes to #49237

[ghost]

I couldn't figure out the best area label to add to this PR. If you have write-permissions please help me learn by adding exactly one area label.

[lambdageek]

LGTM

[lambdageek]

The main feedback is that the design document should be more affirmative: "this is what the runtime does", not "this is what the runtime should do".

Also I think we don't need to talk about the implementation too much (ie, exactly when we parse the file, etc), but we should talk much more about the contract: the name of the task, the names of the parameters, etc.

It would be good to add an example of how to use the task. Something like this (but with correct details):

<!-- FIXME: I'm not sure what the task name is... use the correct one -->
<UsingTask TaskName="GenerateRuntimeConfigBlobTask"... /> <!-- here we should show how to find the task inside the runtime pack -->

<Target Name="GenerateRuntimeConfigBlob" AfterTargets="XYZ"> <!-- what's the right target to run after/depend on? -->
  <GenrateRuntimeConfigBlobTask InputFile="..." OutputFile="..." ReservedProperties="@(...)" />
</Target>

And we should describe the behavior of monovm_runtimeconfig_initialize and how embedders should interact with it. Maybe also with a concise example.

[lambdageek]

just a few nits