v0.8.0

chore: release 0.8.0 (#178)

:robot: I have created a release \*beep\* \*boop\* 
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## [0.8.0](https://www.github.com/googleapis/dialogflow-python-client-v2/compare/v0.7.2...v0.8.0) (2020-03-26)


### Features

* **dialogflow:** add `MediaContent`,  `BrowseCarouselCard`, `ColumnProperties`in v2; add `SpeechContext`, `SpeechWordInfo`in v2; add `enable_word_info`, `speech_contexts`,`model` to InputAudioConfig in v2; add `subtitles` to `Intent.Message.ListSelect` in `v2beta1`; add `language_code` to `ListKnowledgeBase` in v2beta1; add `webhook_headers` to `QueryParameters` in v2beta1 ([#175](https://www.github.com/googleapis/dialogflow-python-client-v2/issues/175)) ([713846b](https://www.github.com/googleapis/dialogflow-python-client-v2/commit/713846b7ed99eaf78cdf383aa9d39b43731b9a0d))
* add validation support to v2; add `output_audio_config_mask` to `detect_intent` method in v2beta1 and v2; add sub agent to v2beta1 (via synth) ([#179](https://www.github.com/googleapis/dialogflow-python-client-v2/issues/179)) ([5a6f18e](https://www.github.com/googleapis/dialogflow-python-client-v2/commit/5a6f18e362b5dd87affbe75e0d0bfc0c21ab87a9))
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This PR was generated with [Release Please](https://github.com/googleapis/release-please).