Move runtime to be inside the toolchain

I am trying to use `rules_cuda` with a hermetic toolchain (e.g. my cuda stuff is not from the system but from some external workspace, whose BUILD file I am currently writing manually based on the prior art in this repo). Among other issues which are so far easily locally patched out I am wondering why the runtime is a flag instead of part of the toolchain. I think static/dynamic should be a target setting and let toolchain selection decide whether we are using the dynamic or static runtime. Then pass `--@rules_cuda//:runtime_static` or something to switch to static. (This means we would register 2 toolchains instead of 1).

Without this it is a pain to support multiple toolchains / cuda versions because selecting the cuda version will also always require passing a secondary flag to pick a runtime for it.

The issue names no files or tests. Start by tracing how runtime selection and CUDA toolchain registration are currently represented in rules_cuda, then compare the manual external-workspace BUILD setup described in the report. Done means toolchain selection chooses the static or dynamic runtime without a separate runtime flag and supports multiple CUDA versions.

Written by the indexing model from the issue text.