[tcgc] refine usage in tcgc

[tadelesh]

idea is from this pr.
we should separate tcgc's usage into two layers:

1. for first layer, we just keep input, output usage which is more native.
2. for second layer, we separate e.g. output to something like the following. It is for client specific output usage.

enum OutputUsage {
  Normal,
  LroInitial,
  Error,
  ...
}

that will be a breaking change, but much extensible and we should make sure each layer orthogonal.
we need to figure out a good data structure to store multi-layer of usages. it must be easy to use.

[iscai-msft]

I do think this is a better way to deal with usage, then the current ballooning we have