Clarify key format

Preamble

I'm filing this issue as suggested by @trishankkarthik inside of this conversation. The comments relevant to this issue are this one and this one.

The issue

The TUF specification states that ecdsa-sha2-nistp256 keys should be PEM encoded (see here):

The "ecdsa-sha2-nistp256" format is:

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

PUBLIC
PEM format and a string.

However, the specification also states:

We define three keytypes below: "rsa", "ed25519", and "ecdsa-sha2-nistp256", but adopters can define and use any particular keytype, signing scheme, and cryptographic library.

The question is: are ecdsa-sha2-nistp256 expected to be encoded only with PEM, or can they be encoded with another format (like hex)?

Personally I agree with what @webern wrote on the linked issue, ecdsa-sha2-nistp256 should be encoded only with PEM.

Read the linked discussion and the cited TUF specification sections around the ecdsa-sha2-nistp256 format. Determine whether the specification intends PEM exclusively or permits other encodings, then clarify the specification accordingly once the project reaches a decision.