Avoid unnecessary loading of agents

First beta-1.x release

Merge pull request #1710 from nunit/issue-491

Fix Invalid XML from TestFixtureBuilder

Merge pull request #1689 from nunit/issue-1678

Ensure that latest extension version is selected if multiple versions present

Merge pull request #1681 from east1k/main

Fix issue #1680

Merge pull request #1684 from nunit/issue-1401

Ensure path to exe is quoted when running dotnet agents

Merge pull request #1674 from nunit/issue-1673

Add new interfaces and enable use of launchers internally

Merge pull request #1672 from nunit/port-1354

Eliminate BinaryFormatter

Merge pull request #1671 from nunit/issue-1670

New Package: NUnit.Agent.Core

Merge pull request #1669 from nunit/issue-1668

Move common classes for console output to nunit.common