Make `/features` API accept percentage as float to enable 0.01%

What does this MR do and why?

The integer gives accuracy of 1%. The flipper happily supports fractional percentages allowing to enable ex. 1 in 10k as 0.01%.

This is helpful as part of phased rollout by groupsharding for #345118 (closed). This allows us to enable feature flag for a fractional percentages (0.01% or 0.0001%).

This is implemented as-is .include?('.') to replicate exact behavior being done by Flipper to consider integer vs float differences.

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