Add MP3 encoding in enum for request config

[yonathan06]

• OS: linux
• Node.js version: 16
• @google-cloud/speech version: 4.10.2

Steps to reproduce

google.cloud.speech.v1.RecognitionConfig.AudioEncoding enum does not have MP3 as an option, even though it is a valid encoding (had to add //@ts-ignore to assign MP3 as value) the current enum in the source code:

enum AudioEncoding {
  ENCODING_UNSPECIFIED = 0,
  LINEAR16 = 1,
  FLAC = 2,
  MULAW = 3,
  AMR = 4,
  AMR_WB = 5,
  OGG_OPUS = 6,
  SPEEX_WITH_HEADER_BYTE = 7,
  WEBM_OPUS = 9
}