improve ssml support

[maxbachmann]

this improves the ssml support for the tts. TODO

• since many providers add their custom ssml tags e.g. whispering each tts class should remove all ssml tags it does not support

[synesthesiam]

Is Google the only provider here to support SSML then?

[maxbachmann]

@synesthesiam no this PR already adds support to

• espeak by using the -m flag
• pico tts already supported it but this makes sure it does not use the speak tag around the sentence, since thats not supported
• google by making sure it uses the speak tags since it will not work otherwise
• flite simply does not support it, while festvox at least supports something similar
• mary tts supports MaryXML, but a lot of tags are the same used by ssml http://mary.dfki.de/documentation/maryxml/index.html

so we should remove/convert all tags that are not supported since even tts engines supporting ssml often add their own custom tags that are not supported by other providers

[maxbachmann]

@synesthesiam Is https://github.com/rhasspy/rhasspy-tts-cli-hermes replacing this tts functionalities? Since it does not allow any tts specific pre processing which would be e.g. required to support SSML. And where are online tts going to be placed, since it appears to be only for locally installed tts that are called via a cli. I would like to directly implement the SSML support for our new services, since this repo is going to be replaced by the services soon anyways