Add CookieStore trait and default Jar

[seanmonstar]

This adapts #1105, with a few adjustments. Firstly, as in #1105, this adds a CookieStore trait so that users can customize where cookies are stored and retrieved on the Client. The adjustments include:

• Changing the method signatures of CookieStore so as to need less allocations. It does make implementing them a little more annoying.
• To remove the breaking change, this makes ClientBuilder::cookie_store still take a boolean using cookie_store crate as the implementation, and adds ClientBuilder::cookie_provider which takes a generic Arc<impl CookieStore>.
• To ease probably the most common thing people want to do, which is simply load a few cookies before using the client, the default cookie store is exposed as reqwest::cookie::Jar, and it has a simple add_cookie_str method.

cc @pfernie

[pfernie]

I had just started commenting in #1105 in response to the various issues referencing it recently; so thank you @seanmonstar for picking this up. I think you've addressed the remaining questions I would have had there w.r.t. breaking change & HeaderValue in this PR already.

Do you need any other feedback or changes from me? To me, this PR satisfies the use cases considered, and looks good to me.

Once a release is published for reqwest containing the CookieStore trait I will publish an update to cookie_store which provides the implementation there for users that want to utilize the more involved cookie_provider(Arc<...>) API.

[seanmonstar]

Do you need any other feedback or changes from me?

No, not really. I was tagging you just in case you had thoughts, and so you knew the PR wasn't going to languish forever :)