add atomic iteration option to codegen

[TomFaulkner]

Adds atomic transactions option to change client to transaction iteration.

This and similar for blocking:

ifself._atomic:
print(f"{INDENT}transaction: edgedb.asyncio_client.AsyncIOIteration,", file=buf)
else:
print(f"{INDENT}client: edgedb.AsyncIOClient,", file=buf)

[fantix]

Thanks for the PR! I'm thinking we should probably do something like this instead:

asyncdefmy_query(
executor: edgedb.AsyncIOExecutor,
...
) ->MyQueryResult:
returnawaitexecutor.query_single(...)

So that both AsyncIOClient and AsyncIOIteration (transaction) are supported.