[python] Add support for production signing with Azure Key Vault

[svenkata9]

Signed-off-by: Sankaranarayanan Venkatasubramanian [email protected]

Description of the changes

This change enables production signing with Azure Key Vault's Managed HSM.
gramineproject/gsc#108 has corresponding gsc changes.

Fixes #11 (for AKV)
Fixes #698 (for AKV)

How to test this PR?

• You need to have Azure Subscription with access to Azure Key Vault's Managed HSM.
• Create a 3072-bit RSA key in Azure Key Vault's managed HSM with public exponent of 3 (Please note that only Managed HSM supports setting public exponent) and give permissions to yourself or who will use the key created as 'Managed HSM Crypto User' (az keyvault role assignment).
• gramine-sgx-sign is modified to take a keytype parameter that will use pem for local private key and akv for Azure Key Vault. Please see updated documentation.

This was tested with my Azure Key Vault subscription, and @dimakuv reviewed it as part of our pair review (Thanks, @dimakuv)

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This change is 

[dimakuv]

Reviewable status: 0 of 5 files reviewed, 2 unresolved discussions, not enough approvals from maintainers (2 more required), not enough approvals from different teams (2 more required, approved so far: ), "fixup! " found in commit messages' one-liners (waiting on @svenkata9)

a discussion (no related file):
Gramine community had discussions where to have such cloud-specific and HSM-specific code. It is currently decided to create them in a separate repo (like contrib) not in this core repo.

So I re-created the functionality of this PR as a plugin here: gramineproject/contrib#11. Please check that PR.

@svenkata9 Could you verify that the new PR works correctly? And in general, please review if I recreated it correctly.

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a discussion (no related file):
After we agree on where to have such HSM-specific code (see my other comment), we'll need to update & review the corresponding GSC PR: gramineproject/gsc#108

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[dimakuv]

So I re-created the functionality of this PR as a plugin here: gramineproject/contrib#11. Please check that PR.

That PR was merged, so we can close this PR.