runtimeConfig: { version: "DEFAULT" } is _not_ the default

[david-perez]

By default, generated crates point to runtime crates using a relative path into the smithy-rs repository. However, you can add

runtimeConfig: { version: "DEFAULT" }

in your smithy-build.json and then it will point to the crates in crates.io.

It's super confusing because you have to opt into the "DEFAULT" behavior.

We should make "DEFAULT" the default behavior i.e. not have to add "DEFAULT".

It'd also be nice if we could override specific crate versions e.g. runtimeConfig: { versions: [ { aws-smithy-http-server: "0.30.0-alpha" } ] }. This might prove flexible when we need to tell users to rollback to a previous compatible / non-vulnerable version, instead of them having to wait for us to publish a new version of the runtime crate / us making a code change.