Call delta_generator to get the signature size

The signature size is needed during payload hashing and signing.
We used to sign the update_engine's payload with RSA keys only. In
this case, the signature size always equals the key size. But the
assumption is no longer true for EC keys, whose DER-encoded signature
size is a variant with a maximum size.

Therefore, we always give the maximum signature size to the delta
generator, who then add paddings to the real signature if necessary.
The maximum signature size is calculated by calling the delta_generator
with the new option '--calculate_signature_size'. For custom payload
signers, we also deprecate the '--payload_signer_key_size' and replace
it with '--payload_signer_maximum_signature_size'.

The EC key in the test is generated with:
The EC key in the unittest is generated with the command:
openssl ecparam -name prime256v1 -genkey -noout -out prime256v1-key.pem
openssl pkey -in prime256v1-key.pem -out testkey_EC.key

Bug: 141244025
Test: sign and verify a payload
Change-Id: Ife6e269d8aa3d870405aca20086330f1795e167f