Reorganize GMS factory namespace

[mars-lan]

Is your feature request related to a problem? Please describe.
With the current directory layout, factories for new entities will likely to end up in a brand new namespace and requires editing of https://github.com/linkedin/datahub/blob/master/gms/war/src/main/webapp/WEB-INF/beans.xml#L7 in order to be scanned correctly. See #1656 for an example.

Describe the solution you'd like
All factories should share a common namespace, e.g. com.linkedin.metadata.factories, similar to the rest.li resources (com.linkedin.metadata.resources) so they can be automatically picked up from the DI framework.

[mars-lan]

Another option is to use the @ComponentScan annotation instead of manually specifying the package in beans.xml: https://www.baeldung.com/spring-component-scanning

[liangjun-jiang]

I just went through the whole process and am able to get it working. I am about to detail the process, and send a PR.

[mars-lan]

@loftyet thanks! Look forward to the PR. Feel free to expand https://github.com/linkedin/datahub/blob/master/docs/how/entity-onboarding.md.

[mars-lan]

@liangjun-jiang are you still working on that PR or do you want us to do the refactoring?