feat: allow setting a query comment through a context value

[kylemcc]

This is an enhancement to recently merged comment functionality. This implementation preserves the current functionality, but allows a comment to be set at the time a query is executed.

This can be used like so:

qctx := bun.ContextWithComment(ctx, "traceid:"+traceID)
err := db.NewSelect().
    Model(&things).
    Where("type = ?", t).
    Scan(qctx)

Which, like the Comment() API will produce:

/* traceid:f2e006f2-8494-4ce6-947e-2c77f4c33dc8 */ SELECT * FROM thing WHERE type = 'whatever'

I should note that in this implementation, if a comment is set through a context, it takes precedence over one set on a query using the Comment() API. If the opposite precedence makes more sense, I'm happy to change this.

I know there was some discussion about supporting multiple comments - I'm happy to add some support for that if it's desirable. I think there's good reason to support both mechanisms and have them play nicely together, but maybe that should be discussed separately. For example, the idea of adding a .Comment("descriptive query name") to individual queries, but then being able to combine that with a context that provides a trace ID so that all queries made as part of a given request can be identified. In this case, I would propose that any comment set on the context appear first in the query followed by any set using the Comment() method.

[j2gg0s]

Passing comments through context is a good idea, thank you for your contribution.

1. If both the Context and Comment methods specify a comment, wouldn’t it make more sense to merge them?
2. We can implement the logic in the beforeQuery hook method, right? That way, the intrusion into the code will be minimal.

[kylemcc]

1. If both the Context and Comment methods specify a comment, wouldn’t it make more sense to merge them?

I wouldn't be opposed to merging them - I think there is definitely value there. If I remember correctly, one of the other discussions raised some questions about comment order if multiple comments are specified. So, I think we just need to decide what the proper behavior should be.

2. We can implement the logic in the [beforeQuery](https://github.com/uptrace/bun/blob/master/hook.go#L55) hook method, right? That way, the intrusion into the code will be minimal.

Yes, I think that would be a reasonable way to refactor and DRY up this addition and the existing implementation. Happy to take a pass at that if you'd like.