Remove quotes from Specifier.String()

[n8maninger]

// MarshalText implements encoding.TextMarshaler.
func (uk UnlockKey) MarshalText() ([]byte, error) {
	return marshalHex(uk.Algorithm.String(), uk.Key[:])
}

Causes UnlockKey to JSON encode as "\"ed25519\":48a468d62e48e27ef24707bd1f467476e9605954e201834c659e614b9704d041"

This will likely break a bunch of things in hostd and renterd. Is adding compatibility code for this in core worth it?

[lukechampine]

dang.

Specifiers are annoying because they aren't always simple strings. For example, siad has a handful of specifiers that end in \n, as well as a few that end in types.RuneToString(2). That's why I chose to quote them in the String method. However, I don't see any "annoying" specifiers in core, renterd, or hostd. So I agree that we should remove the quotes, and maybe add a check to NewSpecifier that rejects non-alphanumeric characters.

As for compatibility... yeah, we should make UnmarshalText handle both cases for a while. We can toss it once we're out of beta.