Log Message:
age: update to 1.2.0

A small release to build the release binaries with a more recent Go toolchain,
and to fix a couple CLI edge cases.

The Go module now exposes a plugin package that provides an age plugin client.
That is, Recipient and Identity implementations that invoke a plugin binary,
allowing the use of age plugins in Go programs.

Finally, Recipients can now return a set of "labels" by implementing
RecipientWithLabels. This allows replicating the special behavior of the scrypt
Recipient in third-party Recipients, or applying policy useful for
authenticated or post-quantum Recipients.