Make `sentry_clientside_traces_sample_rate` configurable via admin panel

What does this MR do and why?

As part of #407169 (closed) we want to expose sentry_clientside_traces_sample_rate to be configurable via admin panel, so we can configure Sentry's client side trances sample rate on the fly (PoC in !114281 (closed)). The sample rate is a value between 0 and 1. So we added:

1. Frontend validation with min: 0, max: 1, step: 0.001
2. Backend validation (in the DB, code !128336 (merged))

Screenshots or screen recordings

Before	After	Backend Validation

How to set up and validate locally

1. Make sure to enable: Feature.enable(:configure_sentry_in_application_settings)
2. In the browser
   1. Login as root or any admin user, in your local GDK.
   2. Go to this Admin Page: https://gdk.test:3443/admin/application_settings/metrics_and_profiling
   3. Update the value
3. Go to rails console
   1. Check the value has been updated: Gitlab::CurrentSettings.sentry_clientside_traces_sample_rate
4. Alternativelly, apply the change in !130144 (merged) to your local GDK and check gon.sentry_clientside_traces_sample_rate in the browser dev tools

MR acceptance checklist

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