Better distinguish between keytype and scheme for ECDSA keys

[adityasaky]

Description of issue or feature request:

The key format provided by securesystemslib defines a keytype field and a scheme field. Currently, there is no distinction between these fields for ECDSA keys.

Current behavior:

ECDSA keys have the following format:

{
    "keytype" : "ecdsa-sha2-nistp256",
    "scheme" : "ecdsa-sha2-nistp256",
    "keyval" : {"public" : PUBLIC}
}

As noted in 761aded, ecdsa-sha2-nistp384 is also supported by securesystemslib.

Expected behavior:

Update securesystemslib to use just ecdsa as keytype while retaining the current scheme formats. This also aligns with the key format for RSA keys where we have:

{
    "keytype" : "rsa",
    "scheme" : "rsassa-pss-sha256",
    "keyval" : {"public" : PUBLIC}
}

[shibumi]

Good work, I just want to mention, that in in-toto-golang we currently follow the expected behavior:

{
    "keytype" : "ecdsa",
    "scheme" : "ecdsa-sha2-nistp256",
    "keyval" : {"public" : PUBLIC}
}

[lukpueh]

Closed by #267