Handle rust-client.* configuration value changes

[Xanewok]

Currently we lazily initialize the configuration once, at extension startup and later we don't update it, even on related rust-client.* configuration value updates.
Probably lazily initializing it from initial user settings is good, but we need to also update changing the values on the go (e.g. changing toolchain used), see #220 (comment).

[chuck-flowers]

I think I have a solution written up now that calculates each of the properties lazily each time the value is read.

This is my first OSS contribution so I was wanting a little guidance with how to submit the pull request. The work is currently in a local branch on my computer that points to a forked version of this repo. I'm assuming I should do the following steps.

1. Make sure my changes are committed to my local branch
2. Push the local branch to my fork
3. Make a pull request for the branch in my fork to a branch in this repo

My question, is which branch should I make the pull request for? Sorry to have to this basic procedural question.

[Xanewok]

You’re right, that’s how the process looks like. The PRs should be done against the master branch here.

GitHub also makes it easier to contribute in general - once you push your branch to your fork, you’ll see a yellow message, on either your fork’s or this repo’s main github site, along the lines of: „You recently pushed chuck-flowers/branch_name” - it also exposes a handy button that can create the initial PR against the upstream repo. The only thing you’ll have to do is fill the PR message.

[chuck-flowers]

Ok thanks. I'll finish testing and make the PR shortly.