Add RPC API for getting rate limits information

[ales-otf]

Description

This PR adds RateLimitInfoRuntimeApi, which provides RPC methods to fetch information about rate-limited transactions. The intention behind it is to allow clients to build logic not based on request errors, but by actual values.

Related Issue(s)

• Closes https://github.com/opentensor/subtensor/issues/892

Type of Change

• [ ] 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):

Breaking Change

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

Checklist

• [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

[sam0x17]

bit late to this but idea: what if we just provide rate limiting information in the responses to all rate-limited requests. Like tell them how close to the quota they are at all times. Then we won't have extra traffic of them constantly checking these rpcs