Alerting: Add MissingSeriesEvalsToResolve to the APIs (#102150)

What is this feature?

A follow-up for #101184, adds AlertRule.MissingSeriesEvalsToResolve to the APIs.

missing_series_evals_to_resolve must be specified too and it must be > 0.

POST /api/ruler/grafana/api/v1/rules/{folderUID} works in the following way:

    If missing_series_evals_to_resolve is not sent or null, the rule keeps its existing value
    If missing_series_evals_to_resolve > 0: updates to that value
    If missing_series_evals_to_resolve = 0: resets to default (nil).
    AlertRule.MissingSeriesEvalsToResolve can't be 0, so I used it to reset

In the Provisioning API, the value is just set if present and > 0. Otherwise it's reset:

PUT to /api/v1/provisioning/alert-rules/{UID}:

    If missing_series_evals_to_resolve is nil, it's reset to the default value
    If missing_series_evals_to_resolve > 0, it's updated

pkg/services/ngalert/api/tooling/api.json

@@ -217,6 +217,10 @@
      },
      "type": "object"
     },
+    "missing_series_evals_to_resolve": {
+     "format": "int64",
+     "type": "integer"
+    },
     "noDataState": {
      "enum": [
       "Alerting",
@@ -1630,6 +1634,10 @@
     "metadata": {
      "$ref": "#/definitions/AlertRuleMetadata"
     },
+    "missing_series_evals_to_resolve": {
+     "format": "int64",
+     "type": "integer"
+    },
     "namespace_uid": {
      "type": "string"
     },
@@ -2864,6 +2872,12 @@
     "metadata": {
      "$ref": "#/definitions/AlertRuleMetadata"
     },
+    "missing_series_evals_to_resolve": {
+     "description": "Number of consecutive evaluation intervals with no data for a dimension must pass\nbefore the alert state is considered stale and automatically resolved.\nIf set to 0, the value is reset to the default.",
+     "example": 3,
+     "format": "int64",
+     "type": "integer"
+    },
     "no_data_state": {
      "enum": [
       "Alerting",
@@ -3150,6 +3164,11 @@
      },
      "type": "object"
     },
+    "missingSeriesEvalsToResolve": {
+     "example": 2,
+     "format": "int64",
+     "type": "integer"
+    },
     "noDataState": {
      "enum": [
       "Alerting",
@@ -4976,6 +4995,7 @@
    "type": "object"
   },
   "gettableAlerts": {
+   "description": "GettableAlerts gettable alerts",
    "items": {
     "$ref": "#/definitions/gettableAlert",
     "type": "object"