Add support for custom ops in declarative builders.

This CL adds support for named custom instructions in declarative builders.
To allow this, it introduces a templated `CustomInstruction` class.
This CL also splits ValueHandle which can capture only the **value** in single-valued instructions from InstructionHandle which can capture any instruction but provide no typing and sugaring to extract the potential Value*.

PiperOrigin-RevId: 237543222