Fix rate limit for setting children

[gztensor]

Description

Add rate limiting to set_childkeys extrinsic that uses passes_rate_limit_globally. Make it specific to subnet.

Matching e2e JS tests:
https://github.com/gztensor/subtensor-js-tests/blob/main/src/childkey.test.js

Related Issue(s)

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Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Other (please describe):

Breaking Change

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Checklist

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have run cargo fmt and cargo clippy to ensure my code is formatted and linted correctly
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

Screenshots (if applicable)

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Additional Notes

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