Add a constant for "empty speak tag"

[sgellock]

We have agreed on a xml tag structure to indicate an empty speak tag. The format of that tag is:

<speak version="1.0" xmlns="https://www.w3.org/2001/10/synthesis" xml:lang="en-US" />

Let's expose this string as a constant in the speech namespace.

For internal user, you can get additional data here:
microsoft/botframework-obi#85

Relates to:
microsoft/botbuilder-dotnet#4311

[enhancement]