Add RateLimit benchmarks

[martincostello]

Details on the issue fix or feature implementation

Add benchmarks for the rate-limit policy added by #903.

This won't work until 7.2.3 is actually released to NuGet.org.

Results from a local run without a baseline are below.

BenchmarkDotNet=v0.13.1, OS=Windows 10.0.19043.1466 (21H1/May2021Update)
Intel Core i9-9980HK CPU 2.40GHz, 1 CPU, 16 logical and 8 physical cores
.NET SDK=6.0.101
  [Host]    : .NET 6.0.1 (6.0.121.56705), X64 RyuJIT
  Polly-dev : .NET 6.0.1 (6.0.121.56705), X64 RyuJIT

Job=Polly-dev  Arguments=/p:BenchmarkFromNuGet=False,/p:SignAssembly=false  

Method	Mean	Error	StdDev	Gen 0	Allocated
RateLimit_Synchronous_Succeeds	59.66 ns	0.593 ns	0.555 ns	0.0296	248 B
RateLimit_Asynchronous_Succeeds	140.89 ns	2.772 ns	2.314 ns	0.0477	400 B
RateLimit_Synchronous_With_Result_Succeeds	49.59 ns	0.429 ns	0.401 ns	0.0191	160 B
RateLimit_Asynchronous_With_Result_Succeeds	107.90 ns	2.142 ns	3.072 ns	0.0191	160 B

Confirm the following

• I started this PR by branching from the head of the latest dev vX.Y branch, or I have rebased on the latest dev vX.Y branch, or I have merged the latest changes from the dev vX.Y branch
• I have targeted the PR to merge into the latest dev vX.Y branch as the base branch
• I have included unit tests for the issue/feature
• I have successfully run a local build