Provide API for adding pseudo headers to HPACKHeaders

Pseudo headers must appear at the beginning of a block of headers. However, there's no API in `HPACKHeaders` to allow for this; it requires up-front knowledge that when constructed the pseudo headers are added first.

When a block of headers is provided (by a user, for example) which must then have pseudo headers added to it, a new block must be created, the pseudo headers added and then all of the headers from the provided block to be copied across. This incurs a potentially large unnecessary extra allocation.

`HPACKHeaders` is currently backed by an array, changing this to a `CircularBuffer` would allow cheap insertion to the front of the block via an `add(pseudoHeader:value:)` API.

Start at HPACKHeaders and inspect its current array-backed storage and header insertion behavior. Confirm that the proposed API preserves pseudo-header-first ordering without rebuilding the supplied block, then validate the behavior with the project's existing HTTP/2 header tests.

Written by the indexing model from the issue text.