Add SQLCL support in Oracle adapter

[walkabout21]

Add support for sqlcl in Oracle adapter by checking for an executable sql binary and defaulting to sqlplus if it doesn't exist.

[walkabout21]

@tpope , just checking if you need any revisions on this. Would have like to have included some kind of "opt out" for sqlcl, but not sure how it could be implemented.

[tpope]

Seeing this code reminded me, all you need to do to opt in is:

let g:dbext_default_ORA_bin = 'sql'

Given that, I think we can probably leave the default as is? Changing the default would mean that Dadbod and dbext would have different defaults, which isn't the worst thing in the world, but also I'm not sure it's worth it just to save one line of configuration.

[walkabout21]

That makes sense. I didn't see anything about that in the docs so I could roll back my code revision and just add the opt in config info to the oracle adapter section of the documentation for the PR?

[walkabout21]

@tpope I rolled back the code change and added a note in the documentation explaining how to override sqlplus as the default binary