Allow to configure additional AWS compute regions in the UI

[r4victor]

#536 implemented backend support for configuring additional AWS compute regions (AWS project configuration now hasextra_regions field). We need to allow users to specify additional regions in the UI when creating/updating AWS projects.

I suggest we add the following multiselect field to the bottom of the form:

**Additional regions**
Select additional regions to run workflows

{multi-seelct}

Info: dstack will try to provision the instance in additional regions if the primary region has no capacity. Specifying additional regions increases the chances of provisioning spot instances.