feat(earn): add function for preparing withdraw transactions

[satish-ravi]

Description

As the title

Test plan

Unit tests, manually by invoking this function

Related issues

• Part of ACT-1180

Backwards compatibility

Yes

Network scalability

If a new NetworkId and/or Network are added in the future, the changes in this PR will:

• Continue to work without code changes, OR trigger a compilation error (guaranteeing we find it when a new network is added)

[codecov]

Codecov Report

Attention: Patch coverage is 90.47619% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 86.27%. Comparing base (db2a063) to head (dc30eeb).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5426      +/-   ##
==========================================
- Coverage   86.27%   86.27%   -0.01%     
==========================================
  Files         753      753              
  Lines       30679    30700      +21     
  Branches     5236     5238       +2     
==========================================
+ Hits        26468    26485      +17     
- Misses       3984     3988       +4     
  Partials      227      227              

Files	Coverage Δ
src/earn/prepareTransactions.ts	95.38% <90.47%> (-2.35%)	⬇️

... and 1 file with indirect coverage changes

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