[labs/context] Allow multiple provider contexts to be added at the same time

Nobody has claimed this yet.

### Should this be an RFC?

- [X] This is not a substantial change

### Which package is this a feature request for?

Context (@lit-labs/context)

### Description

It would be useful to be able to provide multiple contexts at the same time; allowing a provider component to be instantiated the same way but with a dynamic array of contexts

```js
private provider = new MultiContextProvider(this, [
[loggerContext, {
log: (msg) => {
console.log(`[my-app] ${msg}`);
},
}],
[themeContext, myTheme],
[apiContext, myAPI],
]);
```

### Alternatives and Workarounds

It's possible to create a separate provider for every injected service.

Start by reading the @lit-labs/context provider implementation and its existing Provider API. Add support for constructing one provider with an array of context/value pairs, and verify that multiple injected services can be provided together without requiring separate providers.

Written by the indexing model from the issue text.