Support managing keystore entries during deployment creation and update

[dimuon]

Description

Make use of the modified ECE Deployment API that enables specifying ES keystore contents during Create/Update calls.
It should make it possible to handle ES keystore secrets using deployment resource that got a new attribute (example).

The appropriate API change is implemented in ECE 3.7.0, ESS ms-96.
The new IT test (TestAccDeployment_keystore) is expected to fail until ESS ms-96 release. I tested it against my local env.

Related Issues

Rel: #433

Motivation and Context

How Has This Been Tested?

Types of Changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (improves code quality but has no user-facing effect)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation

Readiness Checklist

• My code follows the code style of this project
• My change requires a change to the documentation
• I have updated the documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed

[tobio]

LGTM, some questions to make sure I'm on the same page. I think we can add some comments to the read logic to make this clearer too.

We need to add a changelog entry here.