Offer proguard consumer rules for downstream library consumers.

[d4rken]

Keep rule to prevent code shrinker from breaking validation.

Specifically, proguard/R8 removes values from ExternalParameter due to its private constructor:

dgc-certlogic-android/engine/src/main/java/dgca/verifier/app/engine/data/ExternalParameter.kt

Lines 49 to 58 in b0c6050

	class ExternalParameter private constructor(
	val validationClock: String,
	val valueSets: Map<String, List<String>>,
	val countryCode: String,
	val exp: String,
	val iat: String,
	val issuerCountryCode: String,
	val kid: String,
	val region: String = ""
	) {

Without Proguard: {
    "external": {
        "validationClock": "2021-07-21T15:22:45Z",
        "valueSets": {...},
        "countryCode": "DE",
        "exp": "2022-07-21T07:05:48Z",
        "iat": "2021-07-21T07:05:48Z",
        "issuerCountryCode": "DE",
        "kid": "f1sfUVIx8CA=",
        "region": ""
    },
    "payload": {...}
}

vs

Proguard: {
    "external": {
        "valueSets": {...}
    },
    "payload": {...}
}

Due to the way exceptions are caught, this resulted in a silent fail where a rule evaluation just defaulted to OPEN.