feat: HTTPError.WithFields

[Skn0tt]

This PR adds .WithFields and .WithField methods to HTTPError, which act as a facade to Logrus' methods.
They allow to easily add custom log fields to error messages.

how this helps

At the moment, when you want to add additional information to an HTTP error, you need to print two log lines:

logger.WithError(err).WithField("body", string(body)).Error("error unmarshaling events")
return router.BadRequestError("error parsing body: %s", err.Error())

With this PR, we can trim it down to one:

return router.BadRequestError("error parsing body: %s", err.Error()).WithInternalError(err).WithField("body", string(body))

(taken from https://github.com/netlify/ingesteer/pull/18#discussion_r813652764)

[mraerino]

logrus.Fields is just an alias for a hashmap. multiple entries should be merged, which is the same logrus does for multiple calls to WithFields
you could also use a *logrus.Entry which you could call WithFields on to use their merging logic

[Skn0tt]

I added some naive merging logic, and a test: b692cb1