[client] Name the two config readers for what they do

ReadConfig and GetConfig differed in one thing — what happens when the file is
absent — and neither name said which was which:

- ReadConfig      -> ReadOrGenerateConfig  (reads it, or generates one in memory)
- GetConfig       -> GetExistingConfig     (reads it, or fails)

Three comments went with them:

- GetConfig's said "return with Config and if it was created. Errors out if it
  does not exist", which described a bool it does not return and a creation it
  never performs.
- ReadConfig's explained that it does not write, which is what a reader is
  supposed to do anyway.
- Server.getConfig's said it "errors out if it does not exist", which it does
  not — it resolves a default config, and now provisions the identity too.

client/server/login_gate_test.go

@@ -93,7 +93,7 @@ func TestLogin_ChangeThatBecomesPrivilegedMidRequestHasNoSideEffects(t *testing.
 	require.NoError(t, err)
 	require.Equal(t, profilemanager.ID(activeProfile), active.ID, "the refused login switched the active profile anyway")
 
-	stored, err := profilemanager.ReadConfig(targetPath)
+	stored, err := profilemanager.ReadOrGenerateConfig(targetPath)
 	require.NoError(t, err)
 	require.Equal(t, "https://api.netbird.io:443", stored.ManagementURL.String(), "the refused login moved the management URL")
 }