Enable MERGE strategy for Athena

[jyry-ookla]

As of Athena version 3 it supports MERGE INTO statements for Iceberg tables.

For performance reasons in INCREMENTAL_BY_UNIQUE_KEY it would be good to support using this method to update tables.

I expect this to be a somewhat similar effort to #4985 with the exception that there isn't a partial merge strategy for Athena yet. Probably requires support in SQLGlot.

[matotias]

Same, I was a bit confused with athena not being mentioned in the docs for INCREMENTAL_BY_UNIQUE_KEY