feat: add External ID parameter to AWS converter

[artydeez]

certain credential plugins, like the credential-binding plugin, require that the externalID be specified when using IAM Roles and the aws credential type.

This will add the ability to optionally add iamExternalId in the same way that iamRoleArn is added

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[jtnord]

Thanks for the contribution Ryan.

This looks great, but would you be willing to tweak the doc example and add an extra assert to the unit test?

[jtnord]

Should you add a check that the external id check
is null here?

[artydeez]

@jtnord can you elaborate or add an example? None of the other missing item validations are explicitely checking if that value is null

[jtnord]

https://github.com/jenkinsci/kubernetes-credentials-provider-plugin/blob/master/src/test/java/com/cloudbees/jenkins/plugins/kubernetes_credentials_provider/convertors/OpenstackCredentialv3ConvertorTest.java#L136

[jtnord]

This isn't referenced from the examples page (or elsewhere?
Could you update the existing Aws example with some sommebts for optionality of the fields?

[artydeez]

I don't understand. Do you mean that I should add a reference within the examples README?

[jtnord]

Yes these docs/ will not be visible I documentation unless you reference them somewhere.
https://github.com/jenkinsci/kubernetes-credentials-provider-plugin/blob/master/docs/examples/README.md#aws-credentials

[jtnord]

@rdonahoe-placed do you plan to address the build failure (compilation issue)?

[ERROR] COMPILATION ERROR : 
[INFO] -------------------------------------------------------------
[ERROR] /home/jenkins/workspace/redentials-provider-plugin_PR-57/src/main/java/com/cloudbees/jenkins/plugins/kubernetes_credentials_provider/convertors/AWSCredentialsConvertor.java:[79,15] error: no suitable constructor found for AWSCredentialsImpl(CredentialsScope,String,String,String,String,String,String,String)
     constructor AWSCredentialsImpl.AWSCredentialsImpl(CredentialsScope,String,String,String,String) is not applicable
       (actual and formal argument lists differ in length)
     constructor AWSCredentialsImpl.AWSCredentialsImpl(CredentialsScope,String,String,String,String,String,String) is not applicable
       (actual and formal argument lists differ in length)