Add configurable polling interval for GitHub API calls (#197)

- Add polling-interval-seconds input parameter to action.yaml (default: 10)
- Update polling logic to use configurable interval
- Add input validation to ensure positive values
- Update README with usage documentation

This allows users to customize how frequently the action polls GitHub API
for approval status, enabling them to reduce API calls or speed up response
times based on their needs.

constants.go

@@ -7,7 +7,7 @@ import (
 )
 
 const (
-	pollingInterval time.Duration = 10 * time.Second
+	defaultPollingInterval time.Duration = 10 * time.Second
 
 	envVarRepoFullName                       string = "GITHUB_REPOSITORY"
 	envVarRunID                              string = "GITHUB_RUN_ID"
@@ -25,6 +25,7 @@ const (
 	envVarFailOnDenial                       string = "INPUT_FAIL-ON-DENIAL"
 	envVarTargetRepoOwner                    string = "INPUT_TARGET-REPOSITORY-OWNER"
 	envVarTargetRepo                         string = "INPUT_TARGET-REPOSITORY"
+	envVarPollingIntervalSeconds             string = "INPUT_POLLING-INTERVAL-SECONDS"
 )
 
 var (