[CLI] Add ability to self update binary

[jhnstn]

This adds the ability to self update the cli tool.

Why?

Apple requires code signing for all binaries, making it tricky to download the gbm-cli tool directly from the github releases pages. This change will make it easy for release wranglers to update the tool.

Testing

• Change the version in ./gbm-cli/cmd/root.go to a lower version (current is 1.5.0), example patch:

diff --git a/gbm-cli/cmd/root.go b/gbm-cli/cmd/root.go
index e921336..a7fefb9 100644
--- a/gbm-cli/cmd/root.go
+++ b/gbm-cli/cmd/root.go
@@ -8,7 +8,7 @@ import (
    "github.com/wordpress-mobile/release-toolkit-gutenberg-mobile/gbm-cli/pkg/console"
 )

-const Version = "v1.5.0"
+const Version = "v1.4.0"

 // rootCmd represents the base command when called without any subcommands
 var rootCmd = &cobra.Command{

• Run go build -o ./bin/gbm-cli
• Run ./bin/gbm-cli --version
• Cancel the option to update and note the version number. It should be the value added in the first step
• Run ./bin/gbm-cli --version again
• Accept the offer to update
• Note that the version message shows the latest version in Github
• Run ./bin/gbm-cli --version again
• Note that there is no prompt to update