Add benchmarks for ResponseHeaders sets

[benaadams]

         Method |                 Type |          Mean |     StdDev |           RPS | Allocated |
--------------- |--------------------- |-------------- |----------- |-------------- |---------- |
 BenchmarkAsync |               Common | 1,629.2051 ns | 27.7161 ns |    613,796.25 |       0 B |
 BenchmarkAsync | ContentLengthNumeric |   112.6076 ns |  3.0445 ns |  8,880,393.09 |      32 B |
 BenchmarkAsync |  ContentLengthString |    81.6983 ns |  1.9815 ns | 12,240,157.06 |       0 B |
 BenchmarkAsync |            Plaintext |   192.4687 ns |  2.2937 ns |  5,195,649.48 |       0 B |
 BenchmarkAsync |              Primary |   328.8702 ns |  4.7975 ns |  3,040,713.69 |       0 B |
 BenchmarkAsync |              Unknown |   694.0381 ns |  9.6555 ns |  1,440,843.10 |       0 B |

[cesarblum]

nit: _memoryPool, same for other fields. Or make them properties.

[cesarblum]

Can't you use this https://github.com/aspnet/Logging/blob/dev/src/Microsoft.Extensions.Logging.Abstractions/NullLogger.cs instead?

[cesarblum]

Why benchmark via app instead of just instantiating FrameResponseHeaders and setting directly?

[benaadams]

Superseded by benchmarks in #1308