Consolidate csproj writing

[ArcturusZhang]

Description

Reason of making this change

This change uncovers an issue of the sequence of reading and writing csproj.
The autorest pipeline might parallelize the plugins when they do not have dependencies to each other, in our case, it is csharpgen and csharpproj, they are running in parallel - and in this change, we read csproj in csharpgen, and write csproj in csharpproj which might lead to read file exception because the file is locked to be edited.

We once introduced a partial solution in this issue when we first find the issue with the same root cause, by introducing a CSProjConfiguration class to hold those configurations that are needed when writing csproj file.

Since these configuration should never have a different value from the one when writing source code, and we will never have a concurrent issue when we put them into the same plugin, this class (and its corresponding test cases) is not need therefore removed in this PR.

Changes included in this PR

1. remove the csharpproj plugin (and all the other types and test cases related to it), and move the functionality of this plugin (writes a csproj file) to csharpgen plugin.
2. consolidate the way of writing csproj and other xml document to use XmlWriter instead to make it to have semantics.
3. (WIP) consolidate the writer of writing csproj files into the same class (NewProjectScaffolding class) because we previously have multiple classes doing the same thing.

Follow ups needed to be done after this PR: #4037

Checklist

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