Add basic auth, and default load balancer

[nhaydel]

This PR adds basic auth support for backend services.

Steps to test:

1. Run frontman without any auth config. Ensure apis work as expected.
2. Specify a basic auth config by adding the following config to a backend service:

   auth:
   type: "basic"
   basic:
     username: "test"
     password: "test"

3. Verify that requests return 401 unless a basic auth header is specified with the username and password from the config
4. Repeat steps 2 and 3 with a config specifying env variables:

   auth:
   type: "basic"
   basic:
     usernameEnvVariable: "API_USERNAME"
     passwordEnvVariable: "API_PASSWORD"

note: be sure to set these values in your environment

5. Repeat steps 2 and 3 with a config specifying a credentials file:

   auth:
   type: "basic"
   basic:
     credentialsFile: "credentials.yaml"

   credentials.yaml

   username: "filetest"
   password: "filetest"

[amityahav]

@nhaydel can u add how-to auth in README file?

[hyperioxx]

@nhaydel can u add how-to auth in README file?

Yes please @nhaydel :)

[nhaydel]

@nhaydel can u add how-to auth in README file?

Sorry. Busy weekend. I'll add that this morning. @hyperioxx @amityahav