Provide API for adding pseudo headers to HPACKHeaders

[glbrntt]

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.