Download revocation tails file at issuance time

[TimoGlastra]

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From a comment in the discord:

Can the tails file be cached in AFJ for a given credential at issuance? The file could be as large as 8.4MB according to https://github.com/bcgov/indy-tails-server for a revocation registry of 32768 credentials. If not, the tails server becomes mission critical infrastructure especially for foundational credentials.

We should download the tails fail at issuance time and store it alongside the wallet. Only if there's no local copy of the tails file should we fallback to downloading it from the tails server at proving time.

Note: Currently the filesystem uses the tmp path on both iOS/Android and in Node. We should add a path that provides support for more longer term storage capabilities.