Indexer: add rate-limit configuration to Issuer GraphQL type

[Haroldwonder]

Summary

The Issuer GraphQL type in indexer/src/schema.graphql exposes basic issuer metadata and stats but not rate-limit configuration, even though set_rate_limit events are emitted by the contract.

Acceptance Criteria

• Issuer.rateLimit field added, populated from rate_limit_set events.
• Indexer event handler for rate-limit changes added alongside existing issuer-metadata handling.

[khaylebfortune]

@khaylebfortune has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hello Maintainer, I can fix this issue.

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[Mirabliss]

@Mirabliss has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hello i would like to work on this issue

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Congratulations, @Mirabliss! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

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