Add AppDefaults

[jiaqiluo]

Issue: rancher/rancher#36827

Problem:

We use channelserver to host rke2/k3s version information in rancher/rancher, and we need the ability to set different default versions for different rancher versions.
This ability should be general enough to support any app and not tied to Rancher.

Solution:

This PR adds the support for setting app defaults in the config file.
An example of the value looks like this

appDefaults:
    - appName: rancher
      defaults:
        - appVersion: '> 2.6.2-0 < 2.6.3-0'
          defaultVersion: '1.20.x'
        - appVersion: '> 2.6.2-0 < 2.6.4-0'
          defaultVersion: '1.21.x'
        - appVersion: '> 2.6.4-0'
          defaultVersion: '1.22.x'
    - appName: anotherApp
      defaults:
        - appVersion: '1.2.x'
          defaultVersion: '1.19.5'

Problem:

In the case where the 404 NotFound is returned from the HTTP GET request, channelserver cannot parse the response's data and is stuck in returning errors.

Solution:

This PR adds checks for the response's status code and continues to parse the data only if it is 200.

Problem:

The default value for the flag path-prefix is not applied.

Solution:

It is a bug in the urfave/cli module: urfave/cli#1121
This PR bumps github.com/urfave/cli/v2 to v2.4.0 to get the fix

Others Improvements:

• update error messages to be clear
• update the readme and default config file

[thedadams]

A few minor things.

[jiaqiluo]

@thedadams Please check the latest commit that addresses your comments.

[jiaqiluo]

@thedadams Please check the latest commit: 41efe6c