Performance: Form with external Datasources and IAutofillProvider

Description

When a form is mapped to an external source, the Datasources property in IExternalSystem and GetSuggestedValues method in IAutofillProvider gets called 3 times for every field on the form, when it loads. Though we cache data we fetch from connectors, it adds a lot of overhead when there is a large number of datasources and/or large number of fields on a form.

To address this, we reduced calls to the Datasources property in IExternalSystem and GetSuggestedValues method in IAutofillProvider to only one time for each field, instead of 3 times as is done currently.

We also improved the performance of the IAutofillProvider implementation by getting all suggested values of all fields at the first call then caching them for late use.