chg: dev: add dns_message functions to set EDNS options

The new `dns_message_ednsinit()` and `dns_message_ednsaddopt()` functions
allow EDNS options to be added to a message one at a time; it is no
longer necessary to construct a full array of EDNS options and set
them all at once.

This allows us to simplify EDNS option handling code, and in the
future it wlil allow plugins to add EDNS options to existing
messages.

Merge branch 'each-refactor-message-edns' into 'main'

See merge request isc-projects/bind9!11261