add configuration options to support mtls

[coryb]

This is related to coder/coder#8889

We run the coder api behind a TLS proxy and would like to be able to specify the client cert/key on the vscode plugin requests.

It appears we just need to add cert and key options to the axios default client:
https://github.com/coder/vscode-coder/blob/main/src/extension.ts#L39-L43

I hacked on the plugin a bit it seems these were the only changes necessary:

  const applyInsecure = () => {
    const cfg = vscode.workspace.getConfiguration()
    const insecure = Boolean(cfg.get("coder.insecure"))
    const certFile = String(cfg.get("coder.certFile"))
    const keyFile = String(cfg.get("coder.keyFile"))
    const caFile = String(cfg.get("coder.caFile"))

    axios.defaults.httpsAgent = new https.Agent({
      cert: certFile === "" ? undefined : fs.readFileSync(certFile),
      key: keyFile === "" ? undefined : fs.readFileSync(keyFile),
      ca: caFile === "" ? undefined : fs.readFileSync(caFile),
      // rejectUnauthorized defaults to true, so we need to explicitly set it to false
      // if we want to allow self-signed certificates.
      rejectUnauthorized: !insecure,
    })
  }

I know little typescript, so there is likely a better way to do it, and we would likely want to change the applyInsecure name for the expanded scope.