Introduce profiles_sampler config option similar to traces_sampler

[tadz]

As of v3.15.0 there is an option config profiles_sample_rate and it's value is relative to traces_sample_rate.
I am able to define transactions that have different traces_sample_rate by defining traces_sampler function.
I want to be able to define profiles_sample_rate in same manner by providing profiles_sampler function.

Example:
I've defined traces_sampler to return 0.1 for all transactions (default)
I've defined profiles_sample_rate to return 0.01

I've defined traces_sampler to return 0.0001 for transactionA and transactionB
I want to define profiles_sample_rate to return 1.0 for transactionA and transactionB to send same amount of profiles as traces

these transactions are manually triggered with startTransaction so for now I'll use init function with new options but it would be better to define that as profiles_sampler function