Refactor[BMQEVAL]: fix -WConversion warnings during compilation

[mpak123]

*Issue number of the reported bug or feature request: #87 *

Describe your changes
This is a small good first issue contribution to fix -Wconversion warnings that pollutes the build log.

This particular PR deals with the following warnings in bmq:

/home/runner/work/blazingmq/blazingmq/src/groups/bmq/bmqeval/bmqeval_simpleevaluator.h:970:21: warning: conversion from ‘size_t’ {aka ‘long unsigned int’} to ‘int’ may change value [-Wconversion]
 970 |  return info.d_index; // RETURN

/home/runner/work/blazingmq/blazingmq/src/groups/bmq/bmqeval/bmqeval_simpleevaluator.h:973:29: warning: conversion from ‘size_t’ {aka ‘long unsigned int’} to ‘int’ may change value [-Wconversion]
 973 |  return iter->second.d_index; // RETURN

Testing performed
Observed build logs to confirm that warnings were pruned.

Additional context
Add any other context about your contribution here.

[pniedzielski]

Thanks for your contribution!