Adding custom attribute support for http middleware

[manasachi]

This PR allows extra attributes to be assigned to the logger.

• CustomAttributes struct:

  • If specified, allows users to specify an initializer and assigner function. The initializer should initialize the custom attributes and return then as a map[string]interface{}.
  • At the end of ServeHTTP(), the assigner function takes those initialized attributes as a parameter and updates values as needed

• loggingMiddleware takes source as a field. If specified, it is assigned. Otherwise, the default is "CtxLog"

We can reduce the scope of this PR for context logging (source: CtxLog) and create a folder which allows users to add custom attributes to the logger.