Use JsonMarshal.GetRawUtf8Value instead of JsonElement.GetRawText

[annelo-msft]

There are many places in Azure SDKs where we read and create models using the JsonElement.GetRawText method, for example, here.

Once we're able to upgrade to a version of STJ 9.*, we should be able to improve the performance of these model-reading routines pretty dramatically by calling JsonMarshal.GetRawUtf8Value instead.

@eiriktsarpalis, @m-nash, @jsquire, @KrzysztofCwalina FYI

[eiriktsarpalis]

Great! I will add here the usual disclaimer that JsonMarshal.GetRawUtf8Value is an unsafe method that doesn't handle JSON escaping concerns. If you expect to run this against untrusted input, you need to manually perform an unescaping operation to get the fully normalized result.