Define how keys are derived from secret values for deriveKey

https://www.w3.org/Bugs/Public/show_bug.cgi?id=24811

The deriveKey operation derives a key targeted at a specified
algorithm. Both ECDH and DH algorithms output a Secret Value.

It is not yet specified how to map from the Secret Value to a key for
the specified target algorithm.

It seems intuitive to use the "raw" import format for the target
algorithm with the Secret Value as the raw input. If we do this we
must define how to provide the length of the key and how to convert
the secret value to that length. Presently, raw import for symmetric
keys e.g. AES-GCM derives the key length from the provided data and
fails if the provided data is not one of the supported lengths.

It seems valuable to be able to specify the length of the required key
independently from the length of the Secret Value.

So, one possibility is to allow the length of the symmetric key to be
specified as an input to the import operation and have that operation
define the mapping from arbitrary length raw value to a key of the
requested length. The deriveKey operations can then refer directly to
the "raw" import operations for the derived key algorithm.

...Mark