Document proposed additions to the PSA API: psa_generate_key_ext(),
psa_key_derivation_output_key_ext(). For psa_generate_key_ext(), document
the one use case we intend to implement immediately, which is generating an
RSA key with a custom public exponent.
Subsequent commits will implement the documented functionality.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
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