Minor: allow creating infinite external tables via SQL (for testing)

[alamb]

Which issue does this PR close?

Part of porting tests to sqllogic test #6234

Rationale for this change

Some tests in window.rs programmatically create "infinite" external tables. I want to port them to SQL and thus I need to be able to do the same thing programatically

What changes are included in this PR?

Add support for ('infinite_source', 'true1') in CREATE EXTERNAL TABLE:

CREATE external table t ...
OPTIONS('infinite_source' 'true')
LOCATION 'tests/data/empty.csv';

Add a infinite_source=true in the explain plan (which is super easy after #6202 from @tz70s ❤️ )

logical_plan TableScan: t projection=[c1]
physical_plan CsvExec: file_groups={1 group: [[WORKSPACE_ROOT/datafusion/core/tests/data/empty.csv]]}, projection=[c1], infinite_source=true, has_header=true

Are these changes tested?

yes

Are there any user-facing changes?

I intend this to be used for testing mostly, so I didn't add any entry to the user SQL guide

[alamb]

@mustafasrepo or @ozankabak do you have time to review this small PR? I want to use this feature to simplify tests for infinite stream datasources (that can't be created via sql at this time) including tests in window.rs

[ozankabak]

LGTM code-wise.

BTW, we intended to do this pretty soon -- we were thinking about naming; i.e. thinking about whether it is better UX to say OPTIONS('infinite_source' 'true'), or OPTIONS('infinite' 'true'), or OPTIONS('unbounded' 'true'). Any ideas on this?

[mustafasrepo]

LGTM!. Thanks for this PR @alamb.

[alamb]

BTW, we intended to do this pretty soon -- we were thinking about naming; i.e. thinking about whether it is better UX to say OPTIONS('infinite_source' 'true'), or OPTIONS('infinite' 'true'), or OPTIONS('unbounded' 'true'). Any ideas on this?

I agree infinite_source is not a good name (I chose it to mirror the field in the code). It is also not documented.

In terms of a better UX, I think the ideal UX would be to have some explicit syntax (rather than string name/value pairs). Something like

CREATE external table t ...
INFINITE
LOCATION 'tests/data/empty.csv';

If you agree, perhaps we can merger this PR as is (as a testing tool) and I can file a follow on ticket to make the syntax better.

[alamb]

I filed #6251 to track making a nicer syntax -- I would like to merge this PR to unblock some test updates.