Consistent request header semantics for core code

Currently there are a few headers that are assumed to be present in incoming requests.
Things like `:host`, `:method`, `:scheme` (assumed to be present) and `:path` (assumed to have non zero length), but also more subtle things like `x-forwarded-proto` (assumed to be present).
If not present, the `x-forwarded-proto` is added during the incoming request processing, by the `ConnectionManagerImpl::ActiveStream::decodeHeaders` method.

(I found these headers by running this regex search `ASSERT\(.*headers.*(\.|->)` using ripgrep)

One example of this invariant breaking is in #6417, where a request is rejected (that happens before the headers are added). This invoke encoder filters without the `x-forwarded-proto` headers, which will cause a crash if the encoder filters calls `encoder_callbacks_->route()`.

We should decide on a consistent approach on how to solve this. The following options were discussed in #6417
1. Decide and document which headers are guaranteed to be present in the request headers, and make sure they are added very early in the request processing stage, so that they are always available (even when the request is rejected at an early stage).
2. Decide that there are no guarantees and make sure the code always does null checks before accessing request headers.