feat: [SDK] getLeaders function

[leric7]

Description

We need to use subgraph to get leader details.

Summary of changes

• Removed getAllStakers, and getStaker function from StakingClient.
• Added getLeaders, and getLeader function instead, which reads data from subgraph.
• Removed e2e test of python SDK.

How test the changes

Related issues

Keywords for linking issues

Closes #868

Operational checklist

• All new functionality is covered by tests
• Any related documentation has been changed or added

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[m00n620]

LGTM