Gate the check for gopls updates using a setting

[ramya-rao-a]

As seen in #2768 and #2767, the implicit calls to https://proxy.golang.org/golang.org/x/tools/gopls/@v/list can fail in multiple scenarios.

In VS Code, we also want to follow the guideline of not making any outgoing calls over the network unless it is done as part of a feature resulting from an explicit user action which the user is aware of.

Exceptions to this rule should be documented and a setting should be provided for the user to opt in/out.

Keeping this in mind, the recent feature of making calls to https://proxy.golang.org/golang.org is being updated as follows in this PR

• Provide a setting to opt in to the automatic update check. This is similar to the extensions.autoCheckUpdates setting in VS Code
• If the above is not enabled, then hard-code the latest known latest version of gopls in the extension

[ramya-rao-a]

cc @stamblerre

[stamblerre]

Maybe just:

if config.checkForUpdates {
    const update = await shouldUpdateLanguageServer(tool, path);
    if (update) {
        promptForUpdatingTool(toolName); 
    }
}

[ramya-rao-a]

I'd still want to check against the version that is hard-coded in the extension if the setting is disabled.
How about renaming the setting to go.checkGoProxyForToolUpdates?
For now, we can use it only for gopls.
In the future the same approach can be taken for other tools.