x/tools/gopls: enable telemetry prompting for all gopls-backed editors

[hyangah]

Gopls's telemetry prompting is currently disabled by default.
Client should enable by either setting an internal setting or by calling the prompt command.
VS Code Go extension calls the command for the user selected based on
the hash of vscode client id, after at least a week has passed since a telemetry-enabled
gopls version runs. We now want to move this sampling and decision making to gopls, too,
and enable the prompting in all editors, not only in vscode.

That means we need to record more info in the prompt file in the gopls config directory.

Requirement:

• Preserve the info recorded in the prompt file and avoid prompting users again.
• Do not prompt if the user just started to use gopls. Wait for 1 week.

Some nice-to-haves:

• VS Code Go extension had been running it for awhile. For consistency in sampling, let gopls use the same hash for vs code users.
• If possible, migrate the telemetry start time recorded in the VS Code Go extension's memento.

[gopherbot]

Change https://go.dev/cl/589517 mentions this issue: gopls/internal/server: conditionally prompt for telemetry

[gopherbot]

Change https://go.dev/cl/589836 mentions this issue: internal/settings: prompt for telemetry by default

[gopherbot]

Change https://go.dev/cl/595535 mentions this issue: extension/src/goTelemetry: set telemetry env vars