Add 'PMAT_SCHEDULER_HINT' client calls that can mark code as 'interesting'

[LouisJenkinsCS]

PMAT_SCHEDULER_HINT_FUNCTION - Applies to __FUNCTION__ (perhaps see if I can filter based on ExeContext)

PMAT_SCHEDULER_HINT_START - Dynamically says that this thread is in an interesting block of code

PMAT_SCHEDULER_HINT_STOP - Dynamically says that this thread is no longer in an interesting block of code

This information should influence #58