"ecdsa-sha2-nistp256" wrongly used as default scheme for any "ecdsa" key

[lukpueh]

Description of issue or feature request:
ecdsa has repeatedly been a source of confusion for securesystemslib and its users:
#239, #363, #513, theupdateframework/specification#272, theupdateframework/python-tuf#1388, theupdateframework/go-tuf#508

This is because supported ecdsa signing schemes contain additional details, which either can vary for a given key (hash, e.g. sha2) or cannot (curve, e.g. nistp256).

For comparison, rsa signing schemes only contain details, which can vary (padding, e.g. pss, and hash, eg. sha256).

Current behavior:
There is at least one case, where securesystemslib code uses "ecdsa-sha2-nistp256" as the default scheme for any "ecdsa" key, even if that key uses a different curve:

>>> from securesystemslib.signer import SSlibKey
>>> from cryptography.hazmat.primitives.asymmetric import ec
>>> 
>>> nistp384 = ec.SECP384R1()
>>> private = ec.generate_private_key(ec.SECP384R1())
>>> public = SSlibKey._from_crypto_public_key(private.public_key(), None, None)
>>> public.scheme
'ecdsa-sha2-nistp256'

Expected behavior:

• Fix above
• Bonus: Check if there are other occurrences, where this happens