Add HTTP header version converter

[maskit]

This removes dependency for http2 form http3 by moving their common code into proxy/hdrs/.

The logic is basically unchanged. Only trick I added is a switch to create (reserve) pseudo header fields on HTTPHdr initialization. If you pass HTTP_2_0 or HTTP_3_0 to the init function, MIMEFields for pseudo header fields will be added automatically to ensure those fields will be placed at the beginning of the header field list.

[maskit]

@bneradt Thank you for your feedback. I updated the code and added oxygen comments.

[maskit]

@bneradt I rebased the branch on top of the latest master to resolve conflicts. PTAL.

[bneradt]

Looks good.