support adding multiple connectors to same source_kind, using templates

[eschultink]

Features

• support multiple connectors of same source kind, leveraging the specs as "templates"

TODO:

• solution for cases such as salesforce_domain, which if you want mutliple salesforce instances, you'd need to pass in multiple times to create the "templates"; as it stands, multiple salesforce connector usages via templates will end up with the same salesforce domain.

Change implications

• dependencies added/changed? no
• something important to note in future release notes? no

[eschultink]

1. alter connector "specs" to be more like true templates, and have each evaluated in isolation with a "scope" that's the content of this object??

• problem with that is that I believe Terraform erases/hides object properties not explicitly in variables definition of the module; so we can't have arbitrary content.
• might also make module structure pretty complicated, with connectors being invoked repeatedly

2. have a map(string) that is "merged" over top whatever the spec for the template has by default?
   • not sure this is enough
   • undermines value, bc in several cases you won't re-use that much of the template and will have to re-write a bunch of it.
3. explicit set of properties that can be overridden; target_host, environment_variables, settings_to_provide, and rules_file probably enough ...
   • example_api_requests will remain problematic.