spelling support for various modules

**Is your feature request related to a problem? Please describe.**

```spd-say -o espeak-ng -s hello```

pronounces "hello" instead of spelling it.

Various other modules have the same issue, it seems only the festival, ibmtts and ivona have support for it.

**Describe the solution you'd like**
It should be spelt.

**Additional context**
Possibly adding `spell-out` ssml tags will be enough to get it to work with ssml-powered syntheses such as espeak-ng.
For others, if they don't have an interface for this such as e.g. festival has, we would want to support it by requesting speaking the letters one by one, also see https://github.com/brailcom/speechd/issues/905

No files or tests are named. Start by comparing the existing spelling support in the festival, ibmtts, and ivona modules with the espeak-ng and other module entry points, and review issue #905 for related context. Done means spelling requests work consistently across the affected modules, including the shown espeak-ng example.

Written by the indexing model from the issue text.