Converts legacy integrations config (and the top-level aws config) into
connections and merges them with connections declared explicitly under
connections. Explicit connections take precedence: if a connection type
has any explicit entries, all legacy entries of that type are ignored.
Duplicate legacy entries that resolve to the same connection are dropped,
keeping the first entry, to match the legacy lookup behavior.
The returned connections are fully validated against each connection type's schemas and have default connection and auth method titles assigned.
An InputError is thrown if any part of the configuration is invalid, for
example when a connection fails schema validation or when multiple
connections of the same type resolve to the same identity.
Builds the effective list of connections from configuration.