[Key Vault] Add support for Secure Key Release

[maorleger]

To support Secure Key Release, implement the following changes:

CreateKeyOptions & ImportKeyOptions:

• Add exportable - an optional boolean value
• Add releasePolicy an optional KeyReleasePolicy value (see below)

KeyReleasePolicy

an object / interface containing the following attributes:

• contentType - an optional string. When omitted, defaults to application/json; charset=utf-8 by the service (not in client code)
• data - a byte array encoding the policy rules under which the key can be released.

KeyClient.releaseKey

Used to actually release the key. In JS, we're currently implementing it as follows:

releaseKey(name: string, version: string, target: string, options?: ReleaseKeyOptions): Promise<ReleaseKeyResult>;

where target is the attestation assertion for the target of the key release.

See this PR for implementation: https://github.com/Azure/azure-sdk-for-js/pull/16150/files

Finally, we'll need a resource that can generate an attestation token and return the right keys to MHSM. I set up a test fixture under https://skrattestation.azurewebsites.net/ for now (tracking better implementation in #16314) that you can record against. See our test setup for what is needed there. The important parts are the claims that need to match because those are the claims the test fixture returns.