docs(openapi): Add basic openapi structure

Create basic openapi sctructure for future contributions

Relates moira-alert#44

api/main.yml

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+openapi: 3.0.0
+info:
+  description: "This is an API description for Moira Alert project. Please check https://github.com/moira-alert"
+  version: "2.5.1"
+  title: "Moira Alert"
+  # contact:
+  #   link: "https://t.me/moira_alert"
+  license:
+    name: "MIT"
+servers:
+  - url: http://localhost:8080/api
+    description: Localhost server
+paths:
+  /trigger:
+    $ref: ./triggers/methods.yml

api/triggers/methods.yml

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+put:
+  summary: "Create new trigger"
+  description: ""
+  requestBody:
+    required: true
+    content:
+      application/json:
+        schema:
+          $ref: ./schemas/req_put.yml
+  responses:
+    $ref: ./responses/put.yml

api/triggers/responses/put.yml

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+200:
+  description: "Trigger created"
+  content:
+    application/json:
+      schema:
+        $ref: ../schemas/res_put.yml#/200
+400:
+  description: "Invalid input"
+  content:
+    application/json:
+      schema:
+        $ref: ../schemas/res_put.yml#/400

api/triggers/schemas/req_put.yml

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+$schema: "http://json-schema.org/draft-04/schema#"
+properties:
+    desc:
+        type: string
+        description: Trigger description
+        example: Dice roll trigger
+    error_value:
+        type: number
+        description: Target value at which error will be fired
+        example: 7
+    expression:
+        type: string
+        description: Expression that will be evaluated
+        example: "t1 > 6 ? ERROR : (t1 > 4)? WARN : OK"
+    id:
+        type: string
+    is_remote:
+        type: boolean
+        description: Which storage to use local or remote
+        example: false
+    mute_new_metrics:
+        type: boolean
+        example: true
+    name:
+        type: string
+        description: Trigger name
+        example: Dice Roll
+    patterns:
+        items:
+            type: string
+        type: array
+        example:
+            - local.dice.roll
+        description: Array of possible patterns to check values in simple mode
+    sched:
+        $schema: "http://json-schema.org/draft-04/schema#"
+        $ref: "#/definitions/ScheduleData"
+    tags:
+        items:
+            type: string
+        type: array
+        description: Array of tags associated with this trigger
+        example:
+            - test
+    targets:
+        items:
+            type: string
+        type: array
+        description: Array of targets associated with this trigger
+    throttling:
+        type: integer
+        description: Quantity of events before throttling will be enabled
+    trigger_type:
+        type: string
+        enum: ["rising", "falling", "expression"]
+    ttl:
+        type: integer
+    ttl_state:
+        type: string
+    warn_value:
+        type: number
+        description: Target value at which warning will be fired
+        example: 5
+additionalProperties: false
+type: object
+definitions:
+    ScheduleData:
+        properties:
+            days:
+                items:
+                    $schema: "http://json-schema.org/draft-04/schema#"
+                    $ref: "#/definitions/ScheduleDataDay"
+                type: array
+            endOffset:
+                type: integer
+            startOffset:
+                type: integer
+            tzOffset:
+                type: integer
+        additionalProperties: false
+        type: object
+    ScheduleDataDay:
+        properties:
+            enabled:
+                type: boolean
+            name:
+                type: string
+        additionalProperties: false
+        type: object

api/triggers/schemas/res_put.yml

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+200:
+  type: object
+  properties:
+    id:
+      type: string
+      example: trigger_id
+    message:
+      type: string
+      example: trigger created
+400:
+  type: object
+  properties:
+    status:
+      type: string
+      example: Invalid request
+    error:
+      type: string
+      example: error_value is equal to warn_value, please set exactly one value