feat(v2)!: add ListEnvironments (#167)

BREAKING CHANGE: Resource name usage has changed:
* `ProjectAgentName` is removed. Please use `AgentName` instead.
* `AgentName` previously existed, but was unused. It now matches the behavior of the old `ProjectAgentName` class
* `ContextsClient.update` now requires a `FieldMask` argument.

* Migrate dialogflow from gapic v1 to gapic v2.
Including breaking changes (resource pattern change) introduced in cl/304043500.

committer: @hzyi-google
PiperOrigin-RevId: 305358314

Source-Author: Google APIs <noreply@google.com>
Source-Date: Tue Apr 7 16:00:25 2020 -0700
Source-Repo: googleapis/googleapis
Source-Sha: 83d7f20c06182cb6ada9a3b47daf17b2fd22b020
Source-Link: googleapis/googleapis@83d7f20

* Dialogflow weekly v2/v2beta1 library update:
- Update documentation on various resources.
- Add LRO info to CreateDocument method.

PiperOrigin-RevId: 305810283

Source-Author: Google APIs <noreply@google.com>
Source-Date: Thu Apr 9 19:42:44 2020 -0700
Source-Repo: googleapis/googleapis
Source-Sha: 41d5dee23527df3fc9127838a027df393d79ff3c
Source-Link: googleapis/googleapis@41d5dee

* Dialogflow weekly v2/v2beta1 library update:
- Update comments on Contexts and Sessions resources.

PiperOrigin-RevId: 305919798

Source-Author: Google APIs <noreply@google.com>
Source-Date: Fri Apr 10 12:09:05 2020 -0700
Source-Repo: googleapis/googleapis
Source-Sha: 1e92c343790ae63f88817afde60bd2d5bc775f36
Source-Link: googleapis/googleapis@1e92c34

* feat: Publish ListEnvironments for Dialogflow v2/v2beta1 library.
docs: Clarify WebhookResponse fields for Dialogflow v2/v2beta1 library.

PiperOrigin-RevId: 307849529

Source-Author: Google APIs <noreply@google.com>
Source-Date: Wed Apr 22 10:48:21 2020 -0700
Source-Repo: googleapis/googleapis
Source-Sha: 1d520eaa7bbd8b40f53942ff03b5496fff887b53
Source-Link: googleapis/googleapis@1d520ea

* feat: integrate gapic generator change in googleapis

PiperOrigin-RevId: 308294748

Source-Author: Google APIs <noreply@google.com>
Source-Date: Fri Apr 24 11:44:16 2020 -0700
Source-Repo: googleapis/googleapis
Source-Sha: 28e76243c23cc282efbb288cb558c174e3e5e9ee
Source-Link: googleapis/googleapis@28e7624

* chore: allow breaking change to AgentName class

* chore: allow breaking resource name changes

* chore: allow breaking resource name changes

Co-authored-by: Jeff Ching <chingor@google.com>

google-cloud-dialogflow/clirr-ignored-differences.xml

@@ -0,0 +1,22 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!-- see http://www.mojohaus.org/clirr-maven-plugin/examples/ignored-differences.html -->
+<differences>
+  <!-- TODO: remove after 2.0.0 is released -->
+  <difference>
+    <differenceType>7004</differenceType>
+    <className>com/google/cloud/dialogflow/v2/ContextsClient</className>
+    <method>com.google.cloud.dialogflow.v2.Context updateContext(com.google.cloud.dialogflow.v2.Context)</method>
+  </difference>
+  <difference>
+    <differenceType>7005</differenceType>
+    <className>com/google/cloud/dialogflow/v2/EntityTypesClient</className>
+    <method>* *EntityType*(com.google.cloud.dialogflow.v2.ProjectAgentName*)</method>
+    <to>* *EntityType*(com.google.cloud.dialogflow.v2.AgentName*)</to>
+  </difference>
+  <difference>
+    <differenceType>7005</differenceType>
+    <className>com/google/cloud/dialogflow/v2/IntentsClient</className>
+    <method>* *Intent*(com.google.cloud.dialogflow.v2.ProjectAgentName*)</method>
+    <to>* *Intent*(com.google.cloud.dialogflow.v2.AgentName*)</to>
+  </difference>
+</differences>