Suffix clients with Client after service name

[nateprewitt]

Description of changes:
This is a proposal for adding Client to the end of our generated service clients. While these all originated from the generated namespace from service_name.client import ServiceName, it reads a bit oddly in isolated code snippets.

Prior art from recent SDKs is showing 3 out of the 5 existing Smithy SDKs all suffix with Client to reduce ambiguity. I don't think there's a lot of standing precedent in Python itself for which direction to go. I'm adding this PR as a talking point for if we feel this is necessary.

I don't think it hurts to add this for the moment, but we'll need to decide if we bias one way now, are we're willing to break that based on feedback later?

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[SamRemis]

Since this PR is opened as a point of discussion, just adding my two cents:

Approving this since I'm very much in favor of it. The docs seem to generate fine with this change, but I should probably look more into it than just a quick skim.

Calling this a "client" will make this more familiar for users migrating from botocore and makes it clear that this is not just a general concept, but an object to be used to call the service. I'd be very surprised if we had strong customer feedback making a compelling argument for a different name. If we do, we should make the breaking change before this goes GA.