[Alerting] Enable rule import/export and allow rule types to exclude themselves from export (#102999)

* Removing feature flag changes

* Adding isExportable flag to rule type definition

* Adding isExportable flag to rule type definition

* Adding isExportable flag to rule type definition

* Filtering rule on export by rule type isExportable flag

* Fixing types

* Adding docs

* Fix condition when exportCount is 0

* Unit test for fix condition when exportCount is 0

Co-authored-by: Kibana Machine <42973632+kibanamachine@users.noreply.github.com>

docs/api/alerting/legacy/list.asciidoc

@@ -80,6 +80,7 @@ The API returns the following:
       },
       "producer":"stackAlerts",
       "minimumLicenseRequired":"basic",
+      "isExportable":true,
       "enabledInLicense":true,
       "authorizedConsumers":{
          "alerts":{
@@ -113,6 +114,9 @@ Each alert type contains the following properties:
 | `minimumLicenseRequired`
 | The license required to use the alert type.
 
+| `isExportable`
+| Whether the rule type is exportable through the Saved Objects Management UI.
+
 | `enabledInLicense`
 | Whether the alert type is enabled or disabled based on the license.
 

docs/api/alerting/list_rule_types.asciidoc

@@ -82,6 +82,7 @@ The API returns the following:
       },
       "producer":"stackAlerts",
       "minimum_license_required":"basic",
+      "is_exportable":true,
       "enabled_in_license":true,
       "authorized_consumers":{
          "alerts":{
@@ -115,6 +116,9 @@ Each rule type contains the following properties:
 | `minimum_license_required`
 | The license required to use the rule type.
 
+| `is_exportable`
+| Whether the rule type is exportable through the Saved Objects Management UI.
+
 | `enabled_in_license`
 | Whether the rule type is enabled or disabled based on the license.