feat: Add batch extrinsic support to reduce transaction fees

[bittoby]

Problem

Every stake/unstake operation currently submits a separate on-chain extrinsic. A user staking to 3 netuids across 2 hotkeys pays 6 individual transaction fees. This adds up fast.

Solution

Wrap multiple calls into a single Utility.batch_all transaction so the user only pays one fee regardless of how many operations they're performing.

The core change is a new sign_and_send_batch_extrinsic() method on SubtensorInterface that composes a batch_all call and delegates to the existing sign_and_send_extrinsic. This means proxy, MEV protection, nonce, era, and signing logic are all reused - no duplication.

Batching kicks in automatically when there are 2+ operations. Single operations go through the original path unchanged.

What's batched

• btcli stake add - multiple netuid/hotkey stake operations
• btcli stake remove - multiple netuid/hotkey unstake operations
• btcli stake remove --all - unstake-all across multiple hotkeys

Notes

• batch_all is atomic: all calls succeed or the entire batch reverts. No partial execution.
• Block hash is cached before composing calls to avoid redundant chain queries (per feedback on feat: implement batching for multiple extrinsic calls to reduce trans… #764).

Files changed

• subtensor_interface.py - new sign_and_send_batch_extrinsic() method
• stake/add.py - batch path for multi-operation staking
• stake/remove.py - batch path for multi-operation unstaking + unstake-all
• tests/unit_tests/test_batching.py - 7 unit tests covering the batch method

Test plan

• All 77 unit tests pass
• ruff format and lint clean (no new issues)

Fixes: #567

[bittoby]

@thewhaleking could you review my PR? Welcome to any feedbacks.
Thank you