gpg: Prepare to accept shorter OIDs for ed25519 and cv25519.

* common/openpgp-oid.c (oidtable): Add them.
(oid_ed25519_v5, oid_cv25519_v5): New.
(openpgp_oidbuf_is_ed25519): Take new OID in account.
(openpgp_oidbuf_is_cv25519): Ditto.

ed25519 is used in GnuPG and other implementations since 2015 and thus
we can't simply switch to the shorter OIDs. However, we have not
widely used them with v5 keys (only ed448 forced the use of v5) and
thus it might be possible to use the new OIDs with v5 keys.

Note that Libgcrypt supports the new OIDs even in 1.8.