Support AWS endpoint configuration

[chriskilding]

Allows AWS endpoint configuration (service endpoint, signing region) to be specified globally.

To do

• Make EndpointConfiguration an f:optionalProperty (tick the box in the Jenkins UI if you want to override), and add the necessary tests.
• EndpointConfiguration tests
• Validate the config in doTestEndpointConfiguration()
• Support 'Auto' for signingRegion in doTestEndpointConfiguration()
• Use a service-independent API call to test endpoint connectivity (i.e. not ListSecrets)
• Test locally with a real dependent plugin (probably S3 plugin)

[chriskilding]

aws/aws-sdk-java#2251 says that the AWS API does not support generic authentication tests. We can only test aspects of that (does the URL look well-formed, do the config values look sane etc.)

[jglick]

Is the idea to rework https://github.com/jenkinsci/aws-secrets-manager-credentials-provider-plugin/blob/d64fbd580a8eec4ce5d2b65bd5426fe0acf4bcc4/src/main/java/io/jenkins/plugins/credentials/secretsmanager/config/PluginConfiguration.java to use this?

[chriskilding]

Yes, that was the idea. If client settings can be drawn from this plugin, then we wouldn't need per-plugin configuration for that.