servant-client: Run ClientEnv's makeClientRequest in IO

[Minnozz]

Use cases:

• applyDigestAuth
• Sign requests

[tchoutri]

@Minnozz Hi! Has there been a discussion about this PR? :)

[Minnozz]

@tchoutri Hi! This change was suggested by alp__ on IRC today in response to a question from me. I took a shot at implementing it; should it also be discussed elsewhere?

[tchoutri]

Great, then I'll leave @alpmestan review your PR. :)

[alpmestan]

This looks fine to me. We'll want to add an entry in the relevant changelog. Also, this will force a major version bump.

Let's perhaps leave a few days to give others a chance to give feedback on this, and for you to add a changelog entry (see how other PRs do it, with entries in the changelog.d directory).

[Minnozz]

@alpmestan I added a changelog entry, do you think it needs more explanation?

[alpmestan]

@Minnozz Should be fine, I think people using servant are familiar enough with IO =)

If you ever feel like writing down a little something that demonstrates how you're using this to your advantage (cookbook recipe, blog post, whatever), then it can serve as a reference for users who might need this in the future. But that's more work that adding a few lines in the changelog :-)

[tchoutri]

Considering the approval of this PR, I will merge, but a cookbook or a blog post would be very appreciated, @Minnozz :)