feat(functions): Use flamegraph as data source for slowest functions (#80791)

This switches the slowest functions table within transaction summary to
use flamegraphs as a data source instead of the functions table. This is
because with continuous profiling, the functions table does not have any
transaction information for continuous profiles. So to be compatible
with both profiling modes, we need to switch to using flamegraphs.

Author: Zylphrex