rgw/kms/vault - need libicu to make canonical json for encryption contexts.

rgw/kms/vault - need libicu to make canonical json for encryption contexts.

for encryption, aws s3 provides an "encryption" context to vary per-object
keys.  The encryption context is a base64 encoded json structure, which
must be converted to a determinstic form -- "canonical json".  This
requires converting all strings to a normalized canonical form: "utf-8 nfc",
it also requires thta keys in objects be sorted in a fixed order; so some
form of sorting based on nfc.

It turns out that libicu was the best way to produce utf-8 nfc (boost also
provides a mechanism, but it has many quirks).  So, here are the hooks
to pull the system libicu into the build.

Fixes: http://tracker.ceph.com/issues/48746
Signed-off-by: Marcus Watts <mwatts@redhat.com>