Add a pause command?

**Is your feature request related to a problem? Please describe.**
Some users may want to have a pause e.g. between paragraphs.

This can be implemented several ways:

- using ssml `` and get it implemented for all modules, see https://github.com/brailcom/speechd/issues/905
- adding a pause command in the spd protocol.

At least, for the former we will want to introduce a `pause` message type anyway, so it would probably make sense to make it available through the spd protocol too. Of course it would only be useful at priorities that do not interrupt themselves.

Review the two proposed approaches: SSML support across modules and a pause command in the spd protocol, including the linked speechd issue 905. Determine how a pause message type would fit the protocol and priority behavior. Done means an agreed implementation scope and a pause mechanism usable without self-interruption.

Written by the indexing model from the issue text.