Add support for external bundle validators (#5525)

* add pkg/validate: external validation runner
* add --alpha-select-external flag
* updated external.go to use value from --alpha-select-external
* updated validate.go to use the new --alpha-select-external
* add alpha-select-external to longhelp
* add comment detailing the flag usage.
* remove environment variable
* remove parameters validation stub
* return manifestresults all the way through
* add unit tests for new implementation
* update cli doc 
* return a bool instead of a hard exit.
* avoid reading the entire output of the validator command into memory.
* remove mockable exit var as it is no longer needed

Signed-off-by: jesus m. rodriguez <jmrodri@gmail.com>
Co-authored-by: Eric Stroczynski <ericstroczynski@gmail.com>

internal/cmd/operator-sdk/bundle/validate/cmd.go

@@ -95,7 +95,11 @@ To validate a bundle against the validator for Good Practices specifically, in a
 
 To validate a bundle against the (alpha) validator for Deprecated APIs specifically, in addition to required bundle validators:
 
-  $ operator-sdk bundle validate ./bundle --select-optional name=alpha-deprecated-apis --optional-values=k8s-version=1.22	
+  $ operator-sdk bundle validate ./bundle --select-optional name=alpha-deprecated-apis --optional-values=k8s-version=1.22
+
+To validate a bundle against an external validator, in addition to required bundle validators:
+
+  $ operator-sdk bundle validate ./bundle --alpha-select-external /path/to/external-validator[:/path/to/optional-second-validator]
 	`
 )
 
@@ -139,11 +143,15 @@ func NewCmd() *cobra.Command {
 			if err != nil {
 				logger.Fatal(err)
 			}
-			if err := result.PrintWithFormat(c.outputFormat); err != nil {
+			failed, err := result.PrintWithFormat(c.outputFormat)
+			if err != nil {
 				logger.Fatal(err)
 			}
 
-			logger.Info("All validation tests have completed successfully")
+			// if a test failed don't print that it was successful
+			if !failed {
+				logger.Info("All validation tests have completed successfully")
+			}
 
 			return nil
 		},