Refactor client construction

[sfackler]

Before this PR

Clients were constructed via a new function that took 5 arguments. Configuration updates were handled by creating a RefreshHandle from the client and calling a method on that.

After this PR

==COMMIT_MSG==
Client construction has been refactored to use the refreshables crate and builder-style types.
==COMMIT_MSG==

The new API more closely matches Dialogue, and should be significantly nicer to work with.

• The serde-deserializable config types have had all of their "magic" removed, and map directly to the serialized representation.
• Clients are now constructed via a Builder type to avoid having to pass tons of arguments into a function. The service name and user agent are required, but everything else, including metric registries, is optional, making tests a little bit easier to write.
• There is a new ClientFactory which takes a Refreshable<ServicesConfig> and creates live-updating clients for the services within the configuration. While this is currently very simple and just creates a new client every time, it can be updated in the future to cache clients like Dialogue so that connection pools, node status, etc can be shared.
• Server QoS, service error, and idempotency configuration has been moved out of the client itself and into the builders, and now use enums rather than booleans to allow us to provide more options in the future.

Possible downsides?

Not providing a user agent or service name to the builder results in a panic rather than compile-time error, but that should be something that's caught immediately and fixed if someone forgets.

[changelog-app]

Generate changelog in changelog/@unreleased

Type

• Feature
• Improvement
• Fix
• Break
• Deprecation
• Manual task
• Migration

Description

Client construction has been refactored to use the refreshable crate and builder-style types.

Check the box to generate changelog(s)

• Generate changelog entry

[sfackler]

It's a bit weird to use the conjure-runtime-config types for security and proxy, but unlike Java we don't really have "universal" trust store and proxy selector types to use instead.

[jonsyu1]

Nice, this looks great!