Add an example of channel access token (line#570)

related to line#533 
> channel access token

Author: mokuzon

examples/v2/channel_access_token/Gemfile

@@ -0,0 +1,6 @@
+# frozen_string_literal: true
+
+source "https://rubygems.org"
+git_source(:github) {|repo_name| "https://github.com/#{repo_name}" }
+
+gem 'line-bot-api', path: '../../..' # delete this `path: ...`, and specify the latest version of line-bot-api when you want to use this example out side of this repository.

examples/v2/channel_access_token/Gemfile.lock

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+PATH
+  remote: ../../..
+  specs:
+    line-bot-api (0.0.1.pre.test)
+      base64 (~> 0.2)
+      multipart-post (~> 2.4)
+
+GEM
+  remote: https://rubygems.org/
+  specs:
+    base64 (0.2.0)
+    multipart-post (2.4.1)
+
+PLATFORMS
+  arm64-darwin-21
+  arm64-darwin-22
+
+DEPENDENCIES
+  line-bot-api!
+
+BUNDLED WITH
+   2.4.10

examples/v2/channel_access_token/README.md

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+# Channel access token example
+
+[Issue channel access token](https://developers.line.biz/en/docs/messaging-api/generate-json-web-token/)
+
+## Getting started
+
+```ruby
+$ export LINE_CHANNEL_ID=YOUR_CHANNEL_ID
+$ export LINE_CHANNEL_SECRET=YOUR_CHANNEL_SECRET
+$ bundle install
+$ bundle exec ruby app.rb
+```

examples/v2/channel_access_token/app.rb

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+require 'line-bot-api'
+
+def client
+  # NOTE: Don't need to set `channel_access_token` and `channel_secret` in the client.
+  @client ||= Line::Bot::V2::ChannelAccessToken::ApiClient.new
+end
+
+def main
+  access_token = issue_channel_access_token
+
+  if access_token
+    sleep 1
+    verify_channel_access_token(access_token)
+    revoke_channel_access_token(access_token)
+    verify_channel_access_token(access_token)
+  end
+end
+
+def issue_channel_access_token
+  response = client.issue_channel_token(
+    grant_type: 'client_credentials', # fixed value
+    client_id: ENV.fetch('LINE_CHANNEL_ID'),
+    client_secret: ENV.fetch('LINE_CHANNEL_SECRET')
+  )
+
+  if response.is_a?(Line::Bot::V2::ChannelAccessToken::IssueShortLivedChannelAccessTokenResponse)
+    puts "=== access_token has been issued successfully ==="
+    puts "access_token: #{response.access_token}"
+    puts "expires_in: #{response.expires_in}"
+    puts "token_type: #{response.token_type}"
+
+    response.access_token
+  else
+    puts "=== access_token has not been issued ==="
+    nil
+  end
+end
+
+def verify_channel_access_token(accesss_token)
+  response = client.verify_channel_token(access_token: accesss_token)
+
+  if response.is_a?(Line::Bot::V2::ChannelAccessToken::VerifyChannelAccessTokenResponse)
+    puts "=== access_token has been verified successfully ==="
+    puts "client_id: #{response.client_id}"
+    puts "expires_in: #{response.expires_in}"
+    puts "scope: #{response.scope}"
+  else
+    puts "=== access_token has not been verified ==="
+  end
+end
+
+def revoke_channel_access_token(accesss_token)
+  _body, status_code, _http_headers = client.revoke_channel_token_with_http_info(access_token: accesss_token)
+  if status_code == 200
+    puts "=== access_token has been revoked successfully ==="
+  else
+    puts "=== access_token revoking failed ==="
+  end
+end
+
+main