Add public report extensions.

[branlwyd]

The existing (private) report extension mechanism is renamed to "private extensions."

A few notes:

• Public extensions are always the same for both aggregators; the public extensions are included in the input share AAD to ensure that the Leader can't modify these while passing them along to the Helper.
• Public & private extensions share the same "namespace", i.e. they use the same set of extension type codepoints, and duplicating an extension in both the public & private extensions is considered invalid.

[branlwyd]

Closes #620. +cc @martinthomson

[branlwyd]

Rebased & squashed.