add set_target_stakes_per_interval extrinsic

Description

Add extrinsic to set target stakes per interval

[ ] Bug fix (non-breaking change which fixes an issue)
[x] 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):

If this PR introduces a breaking change, please provide a detailed description of the impact and the migration path for existing applications.

[x] I have performed a self-review of my own code
[x] I have commented my code, particularly in hard-to-understand areas
[x] 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
[x] My changes generate no new warnings
[ ] I have added tests that prove my fix is effective or that my feature works
[x] New and existing unit tests pass locally with my changes
[ ] Any dependent changes have been merged and published in downstream modules

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