settings: add an option to opt out of gopls update prompts completely

[stamblerre]

As @hyangah mentioned on https://golang.org/cl/272209:

Some containers (e.g. google cloud shell IDE or github codespace) are released independently, and they don't want to prompt users for the version update unless it's really critical.

This prevents us from updating the hardcoded gopls versions as frequently. Should we have a setting that turns off the update prompts entirely?

[hyangah]

An alternative option is to repurpose "go.useGoProxyToCheckForToolUpdates".
This is not particularly intuitive - and some users were confused and expected it to prevent the version checks completely.

Or, maybe deprecate and replace it with "go.checkForToolUpdates" which can be off, local, proxy (default). If there are users who don't want to go out to the network, but want to make sure their versions meet the minimum requirements hardcoded in the extension.

[AlbinoGeek]

Please, because often (especially when testing new gopls versions) you have to install them manually, and the periodic tool update check actually offers you to downgrade from the versions you have manually installed, it can be quite obtuse.

[gopherbot]

Change https://golang.org/cl/278353 mentions this issue: package.json: add go.toolsManagement.checkForUpdates

[hyangah]

'"go.toolsManagement.checkForUpdates": "off"` will completely disable any version checking. Not recommended unless it's used in actively maintained dev containers.

Also, when testing new gopls versions manually installed, our extension shouldn't prompt for update if the gopls binary was built in modules mode correctly. If you find otherwise, that's a bug and please file an issue.