Add support for using attribute with type raw

[jobergum]

Several Vespa users use Vespa as a key-value store because of the high availability and elasticity plus the ability to perform ranking for the values retrieved. For low latency serving and with binary payloads, Vespa should support attribute for type raw.

[geirst]

This is now supported in Vespa 8.141.24.

See the system test for examples of use: https://github.com/vespa-engine/system-test/tree/master/tests/search/raw_attributes

[geirst]

Reference doc: https://docs.vespa.ai/en/reference/schema-reference.html#type:raw