Fix the support for phonemes #5822

Please make sure you have searched for information in the following guides.

• Search the issues already opened: https://github.com/GoogleCloudPlatform/google-cloud-node/issues
• Search StackOverflow: http://stackoverflow.com/questions/tagged/google-cloud-platform+node.js
• Check our Troubleshooting guide: https://github.com/googleapis/google-cloud-node/blob/main/docs/troubleshooting.md
• Check our FAQ: https://github.com/googleapis/google-cloud-node/blob/main/docs/faq.md
• Check our libraries HOW-TO: https://github.com/googleapis/gax-nodejs/blob/main/client-libraries.md
• Check out our authentication guide: https://github.com/googleapis/google-auth-library-nodejs
• Check out handwritten samples for many of our APIs: https://github.com/GoogleCloudPlatform/nodejs-docs-samples

A screenshot that you have tested with "Try this API".

<speak>
      <phoneme alphabet="ipa" ph="soso"></phoneme>
</speak>

Link to the code that reproduces this issue. A link to a public Github Repository or gist with a minimal reproduction.

<speak> <phoneme alphabet="ipa" ph="soso"></phoneme> </speak>

A step-by-step description of how to reproduce the issue, based on the linked reproduction.

1. Go to https://console.cloud.google.com/speech/text-to-speech%3Buse-ssml=true%3Binput=%3Cspeak%3E%0D%0A%20%20%20%20%20%20%3Cphoneme%20alphabet=%22ipa%22%20ph=%22soso%22%3E%3C/phoneme%3E%0D%0A%20%20%20%20%3C/speak%3E%3Blocale=en-US%3Bvoice=en-US-Standard-F%3Bencoding=LINEAR16%3Bspeed=1%3Blocation=global?hl=en
2. Write that input on SSML
3. Click play button, each time you will heart nothing

A clear and concise description of what the bug is, and what you expected to happen.

In this case, reproduce /soso/ but really I want Text to Speech can reproduce all phonemes, right now it only support a small IPA subset

A clear and concise description WHY you expect this behavior, i.e., was it a recent change, there is documentation that points to this behavior, etc. **

Because I wrote an IPA reader, and if Text to Speech doesn't support it, I can't use Text to Speech