Add Keymaster support for Filecoin Pin

[Flaxscrip]

https://docs.filecoin.io/builder-cookbook/filecoin-pin

The Protocol Labs community is looking for identity technology.

Adding an option to "persist document on Filecoin" when creating a new Agent or Asset operation on IPFS using filecoin-pin could open up some doors. This could be added to the services offered by a commercial gatekeeper operator.

There is a relevant hackathon we could submit to: https://pl-genesis-frontiers-of-collaboration-hackathon.devspot.app/?activeTab=overview

[macterra]

After reviewing the codebase, DID operations aren't a good fit for Filecoin pinning — they're stored in the database (MongoDB/Redis), with IPFS serving only as a transport/backup layer. Paying for Filecoin permanence on data that's already durable in the DB doesn't add much value.

Where filecoin-pin would make sense is for user data uploads (addText, addData) — content stored directly in IPFS with no other copy. If the Kubo node goes down, that data is gone.

Suggestion: narrow the scope to optional Filecoin persistence for user data uploads, offered as a premium feature through Drawbridge (#121). Free-tier users get ephemeral IPFS storage, paid subscribers get Filecoin-backed durability.

Also note: the issue title says "Keymaster" but IPFS integration lives in the gatekeeper and packages/ipfs — Keymaster doesn't touch IPFS directly.