Add variable size prefix for microbench/db_basic_bench.cc KeyGenerator class + improve documentation

[KareemOtoum]

This pr adds an unfinished feature in the KeyGenerator class at microbench/db_basic_bench.cc, removing the hard-coded limit for the prefix size of the key generator and improves the documentation of the class.

if (prefix_num > 0) {
      prefix_size_ = 4;  // TODO: support different prefix_size
    }

Previously, the key generator would accept an optional prefix and force a limit of 4 bytes if one was provided. The changes in this pr make it so that the constructor accepts a size_t prefix_size = 4 as a parameter to represent the desired bytes for the keys prefix. An assert ensures that the size can represent the given size_t prefix_num.

To support this new feature's addition, the Encode function was also changed to accept a size parameter for encoding a variable size key/prefix while preserving endianness.

This change improves flexibility for key generation in benchmarks and tests.

As a next step, I plan to explore increasing the maximum number of keys (max_keys_) that can be generated, which is currently limited by an assert. Feedback on this approach or suggestions for future improvements are welcome!