Improve Etag ToString performance for DefaultFormat

[johnthcall]

Noticed that ETag ToString was calculating IsValidQuotedFormat even when using default format. Moves the calculation inline to improve performance for default formatting.
The below is a benchmark of ToString() on a Etag with value "test1"

Method	Job	Runtime	Mean	Error	StdDev	Ratio	RatioSD
Before	.NET 4.7.2	.NET 4.7.2	32.601 ns	0.7547 ns	2.2251 ns	1.00	0.00
Before	.NET Core 5.0	.NET Core 5.0	18.484 ns	0.4553 ns	0.8210 ns	0.56	0.04
After	.NET 4.7.2	.NET 4.7.2	7.577 ns	0.2393 ns	0.4723 ns	1.00	0.00
After	.NET Core 5.0	.NET Core 5.0	7.268 ns	0.2279 ns	0.5505 ns	0.96	0.09