Add user setting to disable AI features

[justinrobots]

Description

Currently, there is a setting for turning off AI commit generation in GitLens but this setting does not apply to newer AI features that are being added. For users who need to completely turn off AI features in GitLens, we should provide a setting for doing so that does not require changing the admin setting in gitkraken.dev.

• remove current gitlens.ai.generateCommitMessage.enabled setting
• add a new gitlens.ai.enabled setting
• ensure that messaging is correct in all places to clearly explain if this was a user setting or an org setting that disabled the feature.

duplicates #3960

[d13]

I've added a new gitlens.ai.enabled setting that when set to gitlens.ai.enabled: false will disable all GitLens AI features.

[sergeibbb]

@d13

Indeed, commands and AI-related buttons and menu items has been removed:

But there are few commands that has been left: