PCGC_CAX plugin

[Jason-Zhangxin-Chen]

This PR includes the following changes:

• Refactoring to remove duplicate code in the resolution of configuration in plugins.
• Addition of a new PCGC_CAX plugin for autonity round4 games, along with its corresponding tests.
• Enhancement of the plugin configuration file by providing examples and explanations for all possible configurations.

@raj-shekhar1 @cmjc , please double check if the default configuration of CAX plugin is correct:

var defaultConfig = types.PluginConfig{
	Key:                "",
	Scheme:             "https",
	Endpoint:           "cax.devnet.clearmatics.network", // todo: replace it with cax.piccadilly.autonity.org for piccadilly network.
	Timeout:            10,                               //10s
	DataUpdateInterval: 3600,                             //3600s, todo: if there is a rate limit for the CAX service, we'd need to set it properly.
}

[raj-shekhar1]

Default configs look good to me :+1
Thanks @Jason-Zhangxin-Chen

[cmjc]

Hi lgtm. I've left some suggestions on README edits.

[cmjc]

I just found one more typo!

[cmjc]

👍

[lorenzo-dev1]

LGTM