Configure compilation caching without config files

#### Feature

For embedding wasmtime, i.e., usage via a language bindings, it would be nice if the config cache related settings could be provided via API calls, not config files.

#### Benefit

For use cases embedding wasmtime, it might not be desirable to expose the config file to users of the project that's using wasmtime inside. Furthermore, deploying a configuration file, and handling its absolute path is cumbersome for deployment.

E.g. if there's a CLI application that's using wasmtime, and the user would download the executable and run it, we'd either have to

1. create a config in a temp directory on the fly and remove it afterwards, or
2. instruct the user to put a config into the right place

#### Implementation

It would be nice if the compilation cache settings were exposed in the same way other settings are -- sketching in Go, it could be
```go
func (*Config) SetCache(enabled bool)
func (*Config) SetCacheDirectory(path string)
```
etc.

#### Alternatives

Maybe what I'm thinking about here would be abusing the cache system? 🤔 I don't know...