Add condition for identifying external tables

[antonysouthworth-halter]

resolves #1006
docs dbt-labs/docs.getdbt.com/# N/A

Problem

Missing some cases in table-type detection in Athena adapter.

Solution

Add more edge cases. Not a very nice way but I figure KISS suffices for now.

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc) or this PR has already received feedback and approval from Product or DX

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