Add additional extension URI to Arrow Substrait Compiler

[thisisnic]

Fixes #202

See the unskipped function in this PR (which uses the comparison function "gt", which we hadn't previously implemented here) to see this working.

[paleolimbot]

A few notes...I think the "just use the name with strsplit" plan is probably not going to work as simply as I had hoped.

Could you add an explicit test in test-pkg-arrow.R making sure that arrow_substrait_compiler(mtcars)$resolve_function("add", 1, 2) (or something similarly simplified)` resolves the correct URI anchor? Similarly, I think an explicit test of actually evaluating a plan with a function from each URI would be appropriate (I believe there's already an explicit test for a plan with "add").

[paleolimbot]

Suggested change

	R -e "remotes::install_github('duckdb/duckdb@0.6.0', subdir ='tools/rpkg', build = FALSE)"
	R -e "remotes::install_github('cran/duckdb@0.5.0', build = FALSE)"

...maybe?

[paleolimbot]

I just pushed one more try at getting DuckDB to work in CI. I get a clean CMD check from my end, but I do see:

Warning: [compiler.R:31] Undocumented R6 method: extension_uri_anchor

When I run devtools::document(). It would probably be a good idea to document that method.

[paleolimbot]

If that duckdb hack doesn't work, open a ticket and I'll take a look. It would also be nice to cache the Arrow build so that it doesn't have to rebuild for every commit and both of those could be the same ticket (or maybe DuckDB's release of 0.6.0 will solve that problem entirely).

[thisisnic]

I've opened #211 and skipped those tests running if DuckDB isn't installed with Substrait.