shep list & shep serve implementation

[madhusudhand]

Implemented list and serve commands.

shep list: view list of endpoints

Path	Method	Lambda Function	Region
/test/{id}	get	test-api-test-get	us-east-1
/graphql	post	test-api-graphql-post	us-east-1

shep serve:

starts local serverless environment for the defined endpoints.

When an endpoint is called, runs its associated lambda function and returns response.

[madhusudhand]

any comments on this PR?