Add controlled rollout proposal (#6221)

* Add controlled rollout proposal

* Rename Enabled to Get

proposals/001-controlled-rollout.md

@@ -0,0 +1,115 @@
+# Controlled Rollout proposal
+
+## Why
+
+New features are great, everybody loves them. However, new features come by the hand of new code. New code can have bugs 
+or it can have performance regressions.
+
+Features aren't perfect for all users. Sometimes they are perfect for some users and not others. Sometimes they are 
+perfect for some hosts and not others.
+
+Rolling out a feature shouldn't always be a binary choice: enabled/disabled. In an ideal world, all features would be 
+enabled by default, everybody would love them all, and they would work flawlessly for all possible use cases.
+
+We are in the real world, though, which is not ideal. So we should give people running Fleet tools to rollout features 
+slowly, so that they can update infrastructure if needed, or only use a feature within the scope that is useful for 
+them.
+
+This is a proposal on how this tool could look like and work.
+
+## How
+
+We would create a new type of boolean value in our `AppConfig` called `RolloutBoolean`.
+
+`RolloutBoolean` will have a function `Get(h *fleet.Host) bool`. So instead of doing this:
+
+```go 
+if ac.HostSettings.EnableHostUsers {
+    ...
+}
+```
+
+We would do:
+
+```go 
+if ac.HostSettings.EnableHostUsers.Get(host) {
+    ...
+}
+```
+
+In yaml terms, this is what `RolloutBoolean` would be able to parse:
+
+1. Regular true/false, 0/1, yes/no values
+
+```yaml
+---
+apiVersion: v1
+kind: config
+spec:
+  host_settings:
+    enable_software_inventory: false
+```
+
+2. Only enable a feature for certain teams:
+
+```yaml
+---
+apiVersion: v1
+kind: config
+spec:
+  host_settings:
+    enable_software_inventory:
+      default: false
+      overrides:
+        teams:
+        - team1: true
+        - team2: true
+```
+
+2. Enabled for all except a specified team:
+
+```yaml
+---
+apiVersion: v1
+kind: config
+spec:
+  host_settings:
+    enable_software_inventory:
+      default: true
+      overrides:
+        teams:
+        - team1: false
+```
+
+3. Enabled only for specific hosts:
+
+```yaml
+---
+apiVersion: v1
+kind: config
+spec:
+  host_settings:
+    enable_software_inventory:
+      default: false
+      overrides:
+        host_ids:
+        - 3214: true
+```
+
+4. Enabled for hosts on a specific platform (as reported by osquery, not in terms of label membership):
+
+```yaml
+---
+apiVersion: v1
+kind: config
+spec:
+  host_settings:
+    enable_software_inventory:
+      default: false
+      overrides:
+        platforms:
+        - linux: true
+```
+
+The `Get(h *Host) bool` function will use the provided host to define whether the feature is enabled or not based on
+how it's defined in the configuration.