chore: setup for XML deserialization

[bcanseco]

This PR adds an ObjectMapper accessible to all TwiML Builder subclasses. It also annotates the builder's addText method. Both of these changes are necessary for the upcoming XML deserialization support for TwiML (see internal PR).

Checklist

• [x] I acknowledge that all my contributions will be made under the project's license
• [x] I have made a material change to the repo (functionality, testing, spelling, grammar)
• [x] I have read the Contribution Guidelines and my PR follows them
• [x] I have titled the PR appropriately
• [x] I have updated my branch with the main branch
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] I have added the necessary documentation about the functionality in the appropriate .md file
• [x] I have added inline documentation to the code I modified