EPM fixes

[daemitus]

So there's a number of problems that cropped up.

• When an integration is uninstalled outside of terraform, the change is not picked up. A 404 is not returned. Instead, a 200 item.status="not_installed" is. Requiring a switch to ReadPackageWithResponse and updating getschema (again).
• The schema was wrong (again). The entirety of allOf.1 needs to be under .item which is present in allOf.0. All the more reason to get the Pull newer fleet schema #834 to a satisfactory state.
• Uninstall does not return a 404 like most other delete methods, but a 400 message="name is not installed". So switch to DeletePackageWithResponse.
• Since there's only one non-WithResponse method left at this point in all of fleet, switch InstallPackage to it as well. Does all the body reading for us.
• fleet_integration create and update pass respDiags by value instead of pointer, so if a diag is actually added, it doesn't end up back in the caller.
  Actual:

    resource_test.go:76: Step 1/2 error: Error running apply: exit status 1
        Error: Missing Resource State After Create

Expected (using ti_abusech on one of the older stack versions):

    resource_test.go:76: Step 1/2 error: Error running apply: exit status 1
        Error: Unexpected status code from server: got HTTP 500

[daemitus]

Additionally, I need to check if the diags by value issue is also in all the utils/tfsdk.go helper methods.

type ListMeta struct {
	Index int
	Path  path.Path
	Diags diag.Diagnostics
}

type MapMeta struct {
	Key   string
	Path  path.Path
	Diags diag.Diagnostics
}

[tobio]

👍 couple of minor comments on the code.

[tobio]

Love it, 🎉