refactor: Make option types more backwards compatible

[jakoschiko]

We have two option types: ServerFlowOptions and ClientFlowOptions. We might want to add additional fields to them in the future. It would be nice if we were able to do that without a breaking change. So I propose:

• Let's make them #[non_exhaustive]. This allows us to add fields.
• Let's make ClientFlowOptions non-Copy. This allos us to add non-Copy fields.

The Codec types in imap-codec already do this.

Caveat: To my surprise the .. notation does not work with #[non_exhaustive] (maybe in the future, see rust-lang/rust#70564 (comment)). So this doesn't work:

let options = ServerFlowOptions {
  crlf_relaxed: false,
  ..ServerFlowOptions::default()
}

Instead you need to write this:

let mut options = ServerFlowOptions::default();
options.crlf_relaxed = false;

But I'm willing to accept this.

[coveralls]

Pull Request Test Coverage Report for Build 7843530951

• -1 of 1 (0.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 48.33%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/client.rs	0	1	0.0%

Totals
Change from base Build 7784940186:	0.0%
Covered Lines:	492
Relevant Lines:	1018

---

💛 - Coveralls

[duesee]

Sounds good! Thanks!