Add support for collation attributes on older ICU versions

Add support for collation attributes on older ICU versions

Starting in ICU 54, collation customization attributes can be
specified in the locale string, for example
"@colStrength=primary;colCaseLevel=yes".  Add support for this for
older ICU versions as well, by adding some minimal parsing of the
attributes in the locale string and calling ucol_setAttribute() on
them.  This is essentially what never ICU versions do internally in
ucol_open().  This was we can offer this functionality in a consistent
way in all ICU versions supported by PostgreSQL.

Also add some tests for ICU collation customization.

Reported-by: Daniel Verite <daniel@manitou-mail.org>
Discussion: https://www.postgresql.org/message-id/0270ebd4-f67c-8774-1a5a-91adfb9bb41f@2ndquadrant.com