[Synthetics] Persist auto refresh interval settings in local storage

[dominiqueclarke]

When you change the auto refresh interval setting in Synthetics, changes are not persisted.

When you leave the app and return to Synthetics, the settings reset to the default of 1m.

We should persist the setting 1 of two ways:

1. Persist the setting in local storage
2. Persist the setting as part of the Synthetics settings saved object for the space

I tend to think the better option is to persist in local storage, allowing various users to adjust the the setting in a way that is most useful for their workflow.

[elasticmachine]

Pinging @elastic/obs-ux-management-team (Team:obs-ux-management)

[dominiqueclarke]

Closed by #191333