Add a @Symbol to AmazonWebServicesCredentialsBinding

[jglick]

According to existing bindings this seems a reasonable choice. E.g.

withCredentials([aws(credentialsId: '…')]) {
  sh 'aws …'
}

[oleg-nenashev]

One thing to keep in mind is that JCasC plugin is still bad at conflicting symbol resolution (AFAIK). This particular piece of code is right in general, but I am not sure whether we can expect regressions in JCasC.

CC @Casz @timja @MRamonLeon

[timja]

Should be fine, we hit an issue with a symbol collision in job-dsl but that was because the namespace symbol wasn't working correctly. Jesse fixed it in structs I believe.

[jetersen]

I am not too concerned, if need be we would need some filtering for symbols on JCasC's end. JCasC, for instance, could ignore symbols related to CredentialsBinding if such information is available in structs-plugin or perhaps could be added.

[multani]

@escoem Do you think we could have a new release with that change?
That would clean up our Jenkins quite a bit!