A helper method to capture the logic of deciding whether a field
needs to be boxed. This follows the pattern with other methods like
`optional`, and will allow reusing the logic in the upcoming builder
codegen.

The bit in CodeGenerator::append_oneof was pretty much the same,
except the configuration is looked up for the oneof name.

Rearrange the logic so that intermediate values are bound when needed.
If a repeated fields is configured to be boxed, a deprecation warning
will be emitted by the build script.

Co-authored-by: Casper Meijn <casper@meijn.net>