Add compute_shared_memory_aggs used by shared memory groupby

[PointKernel]

Description

This work is part of splitting the original bulk shared memory groupby PR #16619.

This PR introduces the compute_shared_memory_aggs API, which is utilized by the shared memory groupby. The shared memory groupby process consists of two main steps. The first step was introduced in #17147, and this PR implements the second step, where the actual aggregations are performed based on the offsets from the first step. Each thread block is designed to handle up to 128 unique keys. If this limit is exceeded, there won't be enough space to store temporary aggregation results in shared memory, so a flag is set to indicate that follow-up global memory aggregations are needed to complete the remaining aggregation requests.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[davidwendt]

I don't think this will change within a process. I wonder if it could be cached by making the variable static?

[PointKernel]

The return value of this function will be used in another TU as well: https://github.com/PointKernel/cudf/blob/ed9243b83181d15646b46e15e1aa42963131c5f6/cpp/src/groupby/hash/compute_aggregations.cuh#L69

I've updated the code to calculate the available shared memory only once in compute_aggregations and pass available_shmem_size as an argument, rather than repeatedly calling the API.