Allow providers to provide completions based on raw query text

[takemyoxygen]

Adding a way for different providers to provide completions based on raw query text and current cursor.

The motivation is that some drivers can use SQL dialect not supported by Hue. In these cases, Hue fails to provide any completions at all, for example: #1453 .
It seems reasonable to allow these rare drivers to opt out of using Hue and take burden of parsing the query and providing completions by themselves.

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[gjsjohnmurray]

This looks good. Have you verified it by building a driver that uses it?

[takemyoxygen]

@gjsjohnmurray, I didn't build a new driver per se, I modified the postgres one to return some hardcoded completions and it was working fine.

[gjsjohnmurray]

Thanks for the contribution, and sorry for the delay. One tweak suggested to a log message. Then I think we should merge this and watch for a driver developer to leverage it.