[9.1](backport #5176) Fix download_rate as string on checkin handling

[mergify]

What is the problem this PR solves?

Fix fleet-server to properly handle when an agent sends the upgrade downloading detail's download_rate as a string instead of float64.

How does this PR solve the problem?

Fleet-server will parse the string into a float representing bps and re-marshal the metadata.

Design Checklist

• I have ensured my design is stateless and will work when multiple fleet-server instances are behind a load balancer.
• I have or intend to scale test my changes, ensuring it will work reliably with 100K+ agents connected.
• I have included fail safe mechanisms to limit the load on fleet-server: rate limiting, circuit breakers, caching, load shedding, etc.

Checklist

• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in ./changelog/fragments using the changelog tool

Related issues

• Closes Handle schema incompatibility with Elastic Agent <8.16 #5164

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  This is an automatic backport of pull request Fix download_rate as string on checkin handling #5176 done by Mergify.

[prodsecmachine]

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[elastic-sonarqube]

Quality Gate failed

Failed conditions
B Maintainability Rating on New Code (required ≥ A)

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