Please implement phonetic spelling

**Is your feature request related to a problem? Please describe.**
No.
**Describe the solution you'd like**
Some synthesisers in other languages than English, for example IBMTTS in Brazilian Portuguese, have a kind of bug that makes difficult to distinguish between some vowels when spelling. Thus, we need phonetic spelling, i.e. speaking alpha for a, bravo for b, charlie for c, etc. Some clients like Orca and Fenrir implement this feature theirselves, but Speechd-el for example doesn't. Thus, speech-dispatcher should have it too.
**Describe alternatives you've considered**

**Additional context**

No files or tests are named. Start by locating speech-dispatcher's spelling request handling and compare the phonetic-alphabet behavior described for Orca, Fenrir, and Speechd-el. Done means clients can request phonetic spelling so letters are distinguishable with non-English synthesizers such as IBMTTS in Brazilian Portuguese.

Written by the indexing model from the issue text.