Support granular control of specifying runtime crate versions (#1635)

* Support granular control of specifying runtime crate versions

**BREAKING**: after this PR, `in smity-build.json` the path
`rust-codegen.runtimeConfig.version` no longer exists. Instead, a new
field `versions` comes in. It's an object mapping a runtime crate name
to a version string. There is also a special key `DEFAULT`, which is
presetted as detected runtime version but open to override. Crates
without version specified would be set as the same version as which
associated to key `DEFAULT`.

Signed-off-by: Weihang Lo <weihanglo@users.noreply.github.com>