Implements core MediaConnect endpoints (#3718)

* Adds initial models and implements create_flow endpoint.

* Updates README with a Releases section

* Removes unused import

* Adds __init__.py files to MediaLive and MediaConnect test directories.

* Adds list_flows endpoint and tests.

* Adds describe_flow endpoint and tests.

* Adds delete_flow endpoint and tests.

* Adds URLs

* Adds start_flow and stop_flow endpoints and tests.

* Adds tag_resource and list_tags_for_resource endpoints and tests.

* Uses unquote for ARNs

IMPLEMENTATION_COVERAGE.md

@@ -6738,30 +6738,30 @@
 
 ## mediaconnect
 <details>
-<summary>0% implemented</summary>
+<summary>30% implemented</summary>
 
 - [ ] add_flow_outputs
 - [ ] add_flow_sources
 - [ ] add_flow_vpc_interfaces
-- [ ] create_flow
-- [ ] delete_flow
-- [ ] describe_flow
+- [x] create_flow
+- [x] delete_flow
+- [x] describe_flow
 - [ ] describe_offering
 - [ ] describe_reservation
 - [ ] grant_flow_entitlements
 - [ ] list_entitlements
-- [ ] list_flows
+- [x] list_flows
 - [ ] list_offerings
 - [ ] list_reservations
-- [ ] list_tags_for_resource
+- [x] list_tags_for_resource
 - [ ] purchase_offering
 - [ ] remove_flow_output
 - [ ] remove_flow_source
 - [ ] remove_flow_vpc_interface
 - [ ] revoke_flow_entitlement
-- [ ] start_flow
-- [ ] stop_flow
-- [ ] tag_resource
+- [x] start_flow
+- [x] stop_flow
+- [x] tag_resource
 - [ ] untag_resource
 - [ ] update_flow
 - [ ] update_flow_entitlement