[Specmatic] Improve stub handling for binary responses (e.g., PDF)

**Description** I have an API that has **application/pdf** as **return type** for one of the endpoints. Currently, if i generate a stub from an interaction, i have no way to load bytes in the response (using externalValue). I can only type an inline string. This has two effects: - first i cannot correctly mock this interaction, because the calling app will need valid bytes to be sent back. - second, the mime type sent back is incorrect, Specmatic looks at the pattern from the response example to generate the response. This means if Specmatic parses the example body as a StringValue, it will set the return MimeType to text/plain, which isn't compatible with the contract. There are multiple behaviors that adds up to create the mimetype behaviour: - Specmatic creates the example patterns from the example body, and doesn't look at the contract at all. This is why what is a "String format Binary" in the contract turns in a simple String in the example, generating the incorrect m