recovery: enable clients to send encrypted recovery secrets

[daniel-weisse]

Clients may not want to bring the key material needed to decrypt the recovery secrets, or the recovery secrets themselves, onto machines exposed to the internet.
To enable a recovery workflow where the recovery secrets (or the needed key material) are kept on an airgapped machine, this PR adds a new endpoint to the Coordinator, allowing clients to retrieve an RSA public key generated by the Coordinator for the recovery workflow.
Clients can use this public key to encrypt their shares on the airgapped machine, transfer the encrypted share, and upload it to the Coordinator, where it is decrypted inside the SGX enclave.

Proposed changes

• Add a new endpoint /api/v2/recover/public-key to retrieve a public key generated for the recovery
• Add API functions to retrieve the Coordinator's public recovery key and encrypt your recovery secret (share)
• Add CLI command to retrieve the Coordinator's public recovery key

[netlify]

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