Allow filtering builds by event

[julienduchesne]

A user can now pass &event=cron|push|pull_request|...

The WHERE condition is built like ('' = :build_event OR build_event = :build_event) to allow the same query to be used when an event is specified and when it isn't (empty string)
As such, the List and ListRef methods were also merged into one using the same pattern

This is done to support drone/drone-ui#356. I know that the support of filtering was riding on the implementation so this PR can serve as a discussion base
Let me know what you think of this

[CLAassistant]

All committers have signed the CLA.

[bradrydzewski]

I recommend using a query builder here instead:

var buf strings.Builder
buf.WriteString(queryBase)
buf.Writestring("FROM builds WHERE build_repo_id = :build_repo_id")
if params.Event != "" {
  buf.WriteString("AND build_ref = :build_ref")
}
buf.WriteString(`ORDER BY build_id DESC LIMIT :limit OFFSET :offset`)

And modifying the signature so we have more flexibility in the future to add more filtering criteria:

func (s *buildStore) List(ctx context.Context, repo int64, opts core.BuildListOpts) ([]*core.Build, error)

[marko-gacesa]

Hey @julienduchesne, thanks for your contribution. I'm going to work on the changes that @bradrydzewski suggested. I'm going to build them on top of your commit. When I'm done I'll create a new pull request.