Add text interpolation classes

What does this MR do and why?

As the first step of the text interpolation implementation, this MR adds TextInterpolator and TextTemplate classes. These classes are not yet used - that will come in the next MR. Once text interpolation is fully implemented and has been tested under a feature flag, the original Interpolator and Template classes will be removed and these new classes will be renamed.

Relates to #433002 (closed)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #433002 (closed)