Add support for multi-value header routing

[MihaZupan]

Fixes #1494

Matching now treats multiple values as ORs. I.e. Exact/Prefix must match any of the request values.
NotContains means the value must not match any request header.

Values are further split based on the separator , (or ; for Cookie). A single outer pair of quotes is also removed before matching.

This PR is generally slightly faster on simple (single-value) inputs, and (as expected) slower on multi-value ones.
Modes that may observe a perf degradation are Exact and Prefix.
I don't think the difference is prohibitive to the point that having an opt-out switch is needed.

Benchmark scenarios are defined here.

Method	Scenario	Mean	Error	StdDev	Ratio
Old	SimpleContains	292.78 ns	0.957 ns	0.848 ns	1.00
New	SimpleContains	243.82 ns	0.823 ns	0.687 ns	0.83
Old	SimpleExact	158.98 ns	0.529 ns	0.469 ns	1.00
New	SimpleExact	117.40 ns	0.236 ns	0.184 ns	0.74
Old	SimplePrefix	104.09 ns	0.284 ns	0.222 ns	1.00
New	SimplePrefix	97.69 ns	0.512 ns	0.454 ns	0.94
Old	MultipleContains	294.57 ns	1.679 ns	1.402 ns	1.00
New	MultipleContains	291.51 ns	3.079 ns	2.880 ns	0.99
Old	MultipleExact	124.28 ns	0.237 ns	0.185 ns	1.00
New	MultipleExact	175.34 ns	0.626 ns	0.523 ns	1.41
Old	MultiplePrefix	130.89 ns	1.529 ns	1.431 ns	1.00
New	MultiplePrefix	176.16 ns	0.513 ns	0.428 ns	1.35

ToDo: Updating docs with the new behavior