Add Finch.start_link spec and Finch.pool_opt,pool_opts types

[wojtekmach]

In Req I'm adding Req.request(url, finch: opts) where opts are a combination of Finch.build, Finch.request, and Finch.start_link options, see wojtekmach/req#542 (comment).

I'd like to mention pool options by name in the docs because they can't be all mixed and matched (pool options cannot be given when using a named pool).

I think this is the type of single list of options that's then distributed to the three buckets:

[{:name, Finch.name()} | Finch.request_opt() | Finch.Request.build_opt()] |
  [Finch.pool_opt() | Finch.request_opt() | Finch.Request.build_opt()]

(I'm actually not sure if I'm gonna use above type in my docs because it's pretty complicated and so not sure if that useful.)

[wojtekmach]

As an aside, and I vaguely remember this was maybe brought up already?, here's an idea:

1. Add %Finch.Request{opts: []} field
2. Unify Finch.request_opt and Finch.Request.build_opt into just Finch.request_opt

This way, in Finch.request, Keyword.validate!(opts, allowed) would no longer necessarily be in the hot path so to speak (cc @sabiwara), if users pass all the options to Finch.build, what is a left is a no-op Keyword.validate!([], allowed).

And yeah, my type above could become:

[{:name, Finch.name()} | Finch.request_opt()] |
  [{Finch.pool_opt() | Finch.request_opt()}]

That said, it's rather rare for structs to carry options so this needs consideration!