DNSPolicy: Add CEL validation for geo codes with "GEO-" prefix

[mikenairn]

What

Add CEL validations that ensure only DNSPolices that have a valid continent geo code, specified with the "GEO-" prefix, can be created.

Currently accepted values (with "GEO-" prefix):

AWS_CONTINENT_CODE_AFRICA        = "AF"
AWS_CONTINENT_CODE_ANTARTICA     = "AN"
AWS_CONTINENT_CODE_ASIA          = "AS"
AWS_CONTINENT_CODE_EUROPE        = "EU"
AWS_CONTINENT_CODE_OCEANIA       = "OC"
AWS_CONTINENT_CODE_NORTH_AMERICA = "NA"
AWS_CONTINENT_CODE_SOUTH_AMERICA = "SA"

Note: The geo field is not provider agnostic, and provider specific values are expected to be added here, as such any CEL validation must only take effect on values with the "GEO-" prefix.

Related:

• Fix incorrect geo code in DNS docs #1111