Demonstrate single-compilation-unit build

[hanno-becker]

Some build environments, e.g. AWS-LC's FIPS build, require all .c sources to be compiled in one giant .c file.

• Autogenerate an .c file which includes all mlkem-native .c source files, excluding native backend code.
• Demonstrate single-CU build with single config in an example, without native backends.
• Make monolithic .c file standalone by #undef'ing all #define defined within this giant .c file.
• Demonstrate single-CU build with multiple configs in an example, without native backends.

Incorporating native code will be tracked separately.