Support key rollover in IANA trust anchors

[gnarea]

Describe the problem

Trust anchors currently don't have a "validity period", which means that next time IANA initiates a key rollover and we include the new DS RDATA in newer versions of this library, the old DS may eventually remain valid past its expiry in apps running old versions of this library.

Describe the solution you'd like

Add optional inceptionDate and expiryDate fields to our TrustAnchor, and use them:

dnssec-js/src/lib/TrustAnchor.ts

Lines 4 to 9 in ef0b795

	export interface TrustAnchor {
	readonly keyTag: number;
	readonly algorithm: DnssecAlgorithm;
	readonly digestType: DigestType;
	readonly digest: Buffer;
	}

Additional context

I don't think I've seen this feature in other DNSSEC implementations. dnsjava definitely doesn't support it.