Add the ability to disable storage of failed delivery events in EN DA

[ocofaigh]

COS is used in the EN DA for the storage of failed delivery events. However there is no way to opt out of this feature. Can we add a boolean to disable it? If disabled, ensure the cos bucket does not get created too.

[vburckhardt]

also requirement to be able to use an existing cos bucket in the DA (as opposed to the one created by default)

[ocofaigh]

@vburckhardt its already possible to use an existing bucket using the existing_cos_bucket_name input