Transformer Infra sonic yang singleton container support

[ranjinidn]

What are the changes
Transformer changes to support sonic singleton container. The changes involve the following

• Transformer spec generation, SET and GET flow to support sonic yang singleton container support.
• Added singleton container in sonic test yang and the corresponding mappings from oc to sonic singleton container
• Added UT cases to exercise the CRUD and GET cases for oc and sonic URIs

Singleton container refers to YANG container mapped to a singleton table.
e.g.

               container TEST_SENSOR_GLOBAL {
                       container global_sensor {
                                leaf mode {
                                       type string;
                                }
                                leaf description {
                                       type string;
                               }
                       }
               }

The above sonic yang structure translates to the following ConfigDB key -
"TEST_SENSOR_GLOBAL|global_sensor"

Why the changes are required
To support singleton container in community sonic yangs.
sflow app will be supported in mgmt-framework soon which uses the sonic-sflow.yang from community containing singleton container.

How did I verify the changes
Verified using the singleton container in transformer sonic test yang.
Attached are the UT results and log.
transformer.out.txt
transformer.log

PS: The changes are dependent on the CVL changes for a successful write to DB which will be in a separate PR.

As mentioned in PR #89 this PR also addresses the below.
This is the first step towards addressing sonic-net/sonic-buildimage#15183 requirement. We cannot support all yangs from sonic-buildimage/src/sonic-yang-models as of now due to limitations in UMF code generators, cvl & transformer components. They will be enhanced gradually later.