Allos exploits to provide encoder Compat options

Requirement (re)discovered while processing #2964, where a ARCH_CMD encoder would require the powershell command in order to work. Would be neat if exploits can provide a list of "Compat" options to have into account when selecting the encoder.

Verification
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At this moment nothing should change, because any module provides Compat options for the encoder... so just ensure all works as before. Tests:

- [ ] Use msfpayload to generate any payload you can imagine, should work as before
- [ ] Use msfvenom to generate any encoded payload you can imagine, should work as before.
- [ ] Use any exploit you can imagine, should work as before.