Add support for executable flag binary generation

[0xLE]

What problem does this solve?

Being able to have a binary to print (dynamic) flag for challenges that need them.

Proposed solution

Generate a minimal ELF binary dynamically per instance with the flag (or a templatable print content).

See how Berg does it:

• https://github.com/BergCTF/challenge-instance-controller/blob/cb8d2b80c1459239363f9ca5a53749b6a8a217b5/src/resources/configmap.rs#L90
• https://github.com/BergCTF/challenge-instance-controller/blob/cb8d2b80c1459239363f9ca5a53749b6a8a217b5/src/flag/executable.rs
• https://bergctf.github.io/platform/challenge/?h=exec#executable-binary

Workarounds or alternatives

No response