Support reloading database plugins across multiple mounts

[tomhjp]

This builds on #24472 to support reloading database plugins across multiple mounts. The next stage will be to implement a new root-namespace-only endpoint that allows reloading plugins across multiple namespaces, which will leverage the same function but with some updates. At that point, admins in the root namespace will have everything they need to ensure they can reload all instances of a plugin they've just updated in the plugin catalog without having to enumerate all mounts and namespaces.

This also adds a warning to the response if a reload request ever succeeds but without actually reloading a plugin, for example because the plugin wasn't in use, or if the plugin doesn't even exist in the catalog. We report number of reloads instead of explicitly telling users about whether or not the plugin exists to avoid (admittedly fairly low stakes) enumeration attacks on the contents of the catalog.

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Build Results:
All builds succeeded! ✅

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CI Results:
All Go tests succeeded! ✅

[austingebauer]

👍

[tomhjp]

Note: With the latest commit, I also removed the typeExists check, as I realised it's possible for a plugin to be deleted from the catalog while it's still mounted (separately, it would be nice to fix this, but local-only mounts complicate the solution somewhat). The reload will fail in that case, but I think that's the appropriate result to surface for what is an invalid state for the Vault cluster.