Change Feed Processor: Adds backward compatibility of lease store

[ealsur]

Description

Current state

Currently V3 Change Feed Processor is compatible with V2 Change Feed Processor by doing a forward migration of the schema to the schema used in V3.

Lease schema for V2 CFP

{
    "id": "<lease-id>",
    "PartitionId": "<partition-key-range-id>",
    "Owner": "<some-owner>",
    "ContinuationToken": "<some-continuation>",
}

Lease schema for V3 CFP

{
    "id": "<lease-id>",
    "LeaseToken": "<partition-key-range-id>",
    "Owner": "<some-owner>",
    "ContinuationToken": "<some-continuation>",
}

The V3 SDK performs the in-place migration by removing the PartitionId property and replacing it for the future-proof LeaseToken (for then leases can be created in a different granularity level from partitions).

Problem

The problem is that if the user wants to rollback (return to V2) for whatever reason (maybe rollback the deployment), then the schema is not compatible and they are locked in.

Solution

This PR makes it so the SDK will maintain the PartitionId attribute only if it was previously there (coming from V2) so if the user goes back to V2, the V2 CFP can continue processing. The value of PartitionId will be the same as LeaseToken.

Type of change

• New feature (non-breaking change which adds functionality)

Closing issues

Closes #1722

[github-actions]

Please follow the required format: "[Internal] Category: (Adds|Fixes|Refactors) Description"

Examples:
Diagnostics: Adds GetElapsedClientLatency to CosmosDiagnostics
PartitionKey: Fixes null reference when using default(PartitionKey)
[v4] Client Encryption: Refactors code to external project
[Internal] Query: Adds code generator for CosmosNumbers for easy additions in the future.

[github-actions]

Please follow the required format: "[Internal] Category: (Adds|Fixes|Refactors) Description"

Examples:
Diagnostics: Adds GetElapsedClientLatency to CosmosDiagnostics
PartitionKey: Fixes null reference when using default(PartitionKey)
[v4] Client Encryption: Refactors code to external project
[Internal] Query: Adds code generator for CosmosNumbers for easy additions in the future.

All good!

[ghost]

Closing due to in-activity, pease feel free to re-open.