Get required size of signature before signing.

[NIIBE Yutaka]

Hello,

Stef Bon wrote:
> gcry_pk_algo_info can do this (using GCRY_GET_ALGO_SIGNSIZE for
> example)

We can't use the gcry_pk_algo_info API for this purpose, because the
size of signature depends on a key (in general).

For example, RSA-1024 and RSA-4096 have different signature size.
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