Add COS "direct" endpoint support to landing zone

[ocofaigh]

In the ROKS DA, there is a variable here called existing_cos_endpoint_type. It has variable validation to only allow "public," "private" or null, and the variable description mentions The endpoint type to use when accessing the existing COS instance which doesn't really make sense as COS instances don't have endpoints - its the buckets that have endpoints. So I went digging and found this..

• The input is passed between several internal submodules within the repo, and eventually ends up being used for the flow logs and atracker endpoint_type value in the bucket config here which is then used during bucket creation here (nothing to do with COS instance).
• Since the code was written, a new "direct" endpoint was added and should now be used instead of private. It has to be used on regions that have been re-platformed and should probably now be the default (instead of pubic) for landing zone.