assert on verificationMethod "contents of the value MUST be expressed with a header and encoding"

[aljones15]

We need something like this in assertions:

https://github.com/w3c/vc-di-bbs-test-suite/blob/9cd1d21dbaa076da43149f1dd2366c7d28c46c3a/tests/assertions.js#L135-L167

proofValue (this statement concerns verificationMethod)
A string value that contains the base-encoded binary data necessary to verify the digital proof using the verificationMethod specified. The contents of the value MUST be expressed with a header and encoding as described in Section 2.4 Multibase of the Controller Documents 1.0 specification

• cryptosuite test suite should supply the expected encoding and the expected header(s).
• different cryptosuites have bytes for proofValue https://w3c.github.io/vc-di-bbs/#serializebaseproofvalue

data-integrity-test-suite-assertion/assertions.js

Lines 41 to 55 in 3666beb

	export function expectedMultibasePrefix(cryptosuite) {
	const b64urlNoPadSuites = ['ecdsa-sd-2023', 'bbs-2023'];
	if(b64urlNoPadSuites.includes(cryptosuite)) {
	return {
	prefix: 'u',
	name: 'base64url-no-pad'
	};
	}
	return {
	prefix: 'z',
	name: 'base58btc'
	};
	}

data-integrity-test-suite-assertion/assertions.js

Lines 57 to 66 in 3666beb

	export function isValidMultibaseEncoded(rawValue, multibasePrefix) {
	switch(multibasePrefix) {
	case 'z':
	return shouldBeBs58(rawValue);
	case 'u':
	return shouldBeBase64NoPadUrl(rawValue);
	default:
	throw new Error(`Unhandled encoding prefix: ${multibasePrefix}.`);
	}
	}