GODRIVER-3713 Add a CurrentRaw accessor

[FGasper]

GODRIVER-3713

Summary

This adds a simple accessor to fetch the Current document from a Cursor or ChangeStream.

Background & Motivation

This facilitates iteration of these cursor types via an interface.

[mongodb-drivers-pr-bot]

🧪 Performance Results

Commit SHA: 35941cb

The following benchmark tests for version 692869af1b5f710007155424 had statistically significant changes (i.e., |z-score| > 1.96):

Benchmark	Measurement	% Change	Patch Value	Stable Region	H-Score	Z-Score
BenchmarkMultiFindMany	total_bytes_allocated	-16.4889	633456800.0000	Avg: 758530459.2000 Med: 765480688.0000 Stdev: 47137297.3803	0.7953	-2.6534
BenchmarkMultiFindMany	total_mem_allocs	-16.4408	390320.0000	Avg: 467117.8000 Med: 471050.0000 Stdev: 29057.9195	0.7938	-2.6429
BenchmarkMultiInsertSmallDocument	total_mem_allocs	14.1909	2589917.0000	Avg: 2268058.3816 Med: 2237973.5000 Stdev: 118282.9988	0.8179	2.7211
BenchmarkMultiInsertLargeDocument	ns_per_op	10.6423	32376706.0000	Avg: 29262512.6429 Med: 29016895.0000 Stdev: 882637.9610	0.8511	3.5283
BenchmarkMultiInsertSmallDocument	total_bytes_allocated	10.3758	492827912.0000	Avg: 446499989.7582 Med: 445093368.0000 Stdev: 14484709.8802	0.8354	3.1984
BenchmarkMultiInsertSmallDocument	total_time_seconds	10.1455	1.1682	Avg: 1.0606 Med: 1.0466 Stdev: 0.0460	0.7961	2.3397
BenchmarkSmallDocInsertOne	ops_per_second_max	-7.3652	5758.1132	Avg: 6215.9301 Med: 6245.5500 Stdev: 211.8155	0.7525	-2.1614
BenchmarkSingleRunCommand	allocated_bytes_per_op	0.0933	12208.0000	Avg: 12196.6190 Med: 12196.0000 Stdev: 5.4755	0.7561	2.0785

For a comprehensive view of all microbenchmark results for this PR's commit, please check out the Evergreen perf task for this patch.

[mongodb-drivers-pr-bot]

API Change Report

./v2/mongo

compatible changes

(*ChangeStream).CurrentRaw: added
(*Cursor).CurrentRaw: added