exec credential provider: add install hint

This commit adds the ability for users to specify an install hint for
their exec credential provider binary.

In the exec credential provider workflow, if the exec credential binary
does not exist, then the user will see some sort of ugly

  exec: exec: "does-not-exist": executable file not found in $PATH

error message.  If some user downloads a kubeconfig from somewhere, they
may not know that kubectl is trying to use a binary to obtain
credentials to auth to the API, and scratch their head when they see
this error message.  Furthermore, even if a user does know that their
kubeconfig is trying to run a binary, they might not know how to obtain
the binary.  This install hint seeks to ease the above 2 user pains.

Signed-off-by: Andrew Keesler <akeesler@vmware.com>

Kubernetes-commit: 94e2065df2eef3b198942efb156ef6e27abcc6f9

tools/clientcmd/api/v1/types.go

@@ -209,6 +209,11 @@ type ExecConfig struct {
 	// Preferred input version of the ExecInfo. The returned ExecCredentials MUST use
 	// the same encoding version as the input.
 	APIVersion string `json:"apiVersion,omitempty"`
+
+	// This text is shown to the user when the executable doesn't seem to be
+	// present. For example, `brew install foo-cli` might be a good InstallHint for
+	// foo-cli on Mac OS systems.
+	InstallHint string `json:"installHint,omitempty"`
 }
 
 // ExecEnvVar is used for setting environment variables when executing an exec-based

tools/clientcmd/api/v1/zz_generated.conversion.go

@@ -358,6 +358,7 @@ func autoConvert_v1_ExecConfig_To_api_ExecConfig(in *ExecConfig, out *api.ExecCo
 	out.Args = *(*[]string)(unsafe.Pointer(&in.Args))
 	out.Env = *(*[]api.ExecEnvVar)(unsafe.Pointer(&in.Env))
 	out.APIVersion = in.APIVersion
+	out.InstallHint = in.InstallHint
 	return nil
 }
 
@@ -371,6 +372,7 @@ func autoConvert_api_ExecConfig_To_v1_ExecConfig(in *api.ExecConfig, out *ExecCo
 	out.Args = *(*[]string)(unsafe.Pointer(&in.Args))
 	out.Env = *(*[]ExecEnvVar)(unsafe.Pointer(&in.Env))
 	out.APIVersion = in.APIVersion
+	out.InstallHint = in.InstallHint
 	return nil
 }