Move generate_only and simulate to baseReq in REST txs

[fedekunze]

Summary

Move generate_only and simulate from query params to baseReq in POST endpoints

Proposal and Problem Definition

Currently they are set as query parameters, it'd be much simpler to move them within the baseReq with default value false. Then we won't need to parse them or have functions like HasGenerateOnlyArg and HasDryRunArg.

cc: @jackzampolin @alexanderbez @alessio

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[alessio]

Fair enough, I'll take this.

[alexanderbez]

Yup. Seems like a cleaner approach to me. Are you implementing this or should I?