feat: Add API to set custom DefaultConfig.Instance

[filzrev]

This PR add DefaultConfig.SetDefault(config) API to customize default global config.

Background
BenchmarkDotNet use DefaultConfig.Instance when global config is not passed to BenchmarkRunner/BenchmarkSwitcher.

On normarl use case, It can specify global config by using Main entry point.
But when running benchmark via TestAdaper, It can't use Main entry point (and launchsettings.json)
So. it need to customize config via assembly-level ConfigAttribute.
https://benchmarkdotnet.org/articles/features/vstest.html?q=testadapter#setting-a-default-configuration

This PR add DefaultConfig.SetDefault(config) API.
By this change, it can use custom config by TestAdapter, (Settings are loaded from .runsettings/testconfig.json)

[timcassell]

How is it expected to be used? The test adapter supports providing your own entry point by setting <GenerateProgramFile>false</GenerateProgramFile>.

[filzrev]

How is it expected to be used?

It's needed when running benchmarks from VS TestExplorer.

When running benchmark via TestExplorer.
It always use DefaultConfig.Instance because Program.cs entry point is not used.
I want to customize this behavior by this PR.

[timcassell]

It's needed when running benchmarks from VS TestExplorer.

Right, you said that already, but it doesn't explain how.

[filzrev]

Right, you said that already, but it doesn't explain how.

The following code is expected API usage on VSTestAdapter (with VsTest).
filzrev@c438bd3

By this changes, It can specify custom arguments when running benchmark on Test Explorer.

[timcassell]

The name is misleading, it's actually setting Instance.