[RateLimiter] Allow overriding the key of a compound limiter's sub-limiters

A compound rate limiter forwards the same key to every sub-limiter. Mixing
a per-user limiter with a global one in the same compound already works
with a custom RateLimiterFactoryInterface implementation, but cannot be
expressed from framework.rate_limiter configuration, where compound
limiters are actually declared.

- Add an optional $keys argument to CompoundRateLimiterFactory: a map
  of sub-limiter name to a key that always takes precedence over the
  one passed to create(). An unknown name in $keys makes create()
  throw instead of being silently ignored.
- Add an optional "key" config option under
  framework.rate_limiter.<name>.limiters.*, wired to the above. A
  compound referencing an unknown sub-limiter name now throws with
  the offending name(s) instead of a generic message.
- "limiters" keeps accepting a plain list of names: a map form lets
  individual entries set "key". The compound's IteratorArgument in
  the compiled container is now indexed by limiter name instead of
  position.

Individual limiter definitions stay untouched and reusable outside of
compounds.

A fixed key is shared by every caller of the compound. CompoundLimiter::consume()
draining a shared quota on a rejected consume is a pre-existing issue,
left for a separate fix. CompoundLimiter::reset() clearing a shared
sub-limiter's window along with the caller's own is a known limitation
of this feature, to be documented.