Improve extension activation time

[bwateratmsft]

Fixes #2371.

This covers:

• Get Docker ID (this is already a one-time penalty, it's cached from then on)
• Filesystem checks for Docker context paths
• Setting the VSCode contexts
• Don't wait on Docker context loading at all?? Can that be done without substantial refactoring?

In the most common case (no Docker contexts, therefore don't need to run CLI), the remaining time spent on activation is mostly dominated (~75%) by Dockerode loading. Nevertheless that will stay as-is; it was an improvement to lazy load the Docker client provider.

[bwateratmsft]

Don't wait on Docker context loading at all?? Can that be done without substantial refactoring?

I've looked into it, and I think the answer is no. The real need is for ext.dockerClient to be set, since the activation reason might be any of the commands that use it. The simplest way would be to make ext.dockerClient an AsyncLazy but it has dozens of callers so it would be a significant amount of refactoring.