Optional PreImageOracle

[martyall]

Not every mips program requires this, but we currently throw an exception if it isn't provided

[dannywillems]

Approving for now. It is not the best solution as we discussed, but it works, and help to get something working.

We will also need to change it when we integrate the prover/verifier for RISC-V.

Also, we need a better solution to handle MIPS programs that are not targeting OP (i.e. no oracle for instance). We might want to have in the future a different subinterpreter for the syscalls..

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 43 lines in your changes missing coverage. Please review.

Project coverage is 74.66%. Comparing base (84c1103) to head (343132b).
Report is 31 commits behind head on master.

Files with missing lines	Patch %	Lines
o1vm/src/legacy/main.rs	0.00%	15 Missing ⚠️
o1vm/src/pickles/main.rs	0.00%	15 Missing ⚠️
o1vm/src/preimage_oracle.rs	0.00%	11 Missing ⚠️
o1vm/src/test_preimage_read.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2920      +/-   ##
==========================================
+ Coverage   73.44%   74.66%   +1.21%     
==========================================
  Files         259      259              
  Lines       61618    61713      +95     
==========================================
+ Hits        45253    46075     +822     
+ Misses      16365    15638     -727     

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