Hardware security module-based key management solutions are not supported

[bbenligiray]

The user is expected to provide the master key mnemonic for deployment flexibility (https://github.com/api3dao/airnode/issues/242).

As an alternative, allow the user to use a private key that is stored in a hardware security module-based key management service such as AWS KMS. Note that this will practically result in vendor lock-in, and thus should be preferred only by very specific use-cases.

[MrAngus666]

In case we want to implement this for multiple providers, we have:

• GCP: https://cloud.google.com/security-key-management
• Azure: https://azure.microsoft.com/en-us/services/key-vault/

[bbenligiray]

This will be especially useful for HTTP signed data gateway and relayed RRP/PSP users. There is increasing support for ECDSA by cloud providers since this issue was created.

[aquarat]

This task relates: #1443