Add percentage weight to server_groups_list

[jasmine-nahrain]

This adds the option to choose how often a server group is used in sni.

[maskit]

Maybe it's a ratio then?

[maskit]

Or maybe a weight as you say on the subject.

[maskit]

I'm not too sure if this is a good approach. groups_list is probably not the only setting that users would want to configure like this. And some of those may not be controllable on sni.yaml.

I feel like making A/B test plugin is a better approach. I can't think of any reason to have the mechanism on ATS core.

[maskit]

This is what I'm thinking. We'd need to introduce a new TS API to set group_list, but the idea is basically enabling plugins to do something on an SNI action. The difference from a global plugin that uses TS_SSL_SERVERNAME_HOOK would be that the plugin doesn't need to lookup server names. The plugin (TSSNIDoAction) will be called only if ATS core finds a plugin-action for the server name, like remap plugins are called only if there's a match.
#12658

[jasmine-nahrain]

[approve ci autest 1]