Add option download auto

We can have option auto here:
https://github.com/node-gradle/gradle-node-plugin/blob/master/docs/usage.md#configuring-the-plugin

look at the comments

node {
    // options: yes | no | auto
    // Whether to download and install a specific Node.js version or not
    // If "no", it will use the globally installed Node.js
    // If "yes", it will download node using above parameters
    // Note that npm is bundled with Node.js
    // If "auto", it will check and download node only if its mising (auto will check if node is installed and switch between yes and no modes)
   
    download = 'auto'
}

In CI environment there is no sense to download node if its already exist. If node exist and its downloaded another version of node from this plugin its have versions conflicts in alpine:
https://github.com/node-gradle/gradle-node-plugin/issues/51

for this issue with download = true :

#8 141.5 FAILURE: Build failed with an exception.
#8 141.5 
#8 141.5 > Task :webpack FAILED
#8 141.5 * What went wrong:
#8 141.5 Execution failed for task ':webpack'.
#8 141.5 > A problem occurred starting process 'command '/.gradle/nodejs/node-v16.14.0-linux-x64/bin/node''

my temp fix is to set download = false and install node manually apk add --update nodejs npm

Start with the configuring-the-plugin section in docs/usage.md and review the linked issue #51 for the existing version-conflict context. Trace the current download option behavior from that configuration, then confirm that an auto option can use an installed Node.js when present and download the configured version when it is missing.