[GenAPI] Fix header handling in GenAPI and CSharpFileBuilder

[carlossanlop]

Should fix the bug reported in dotnet/source-build-assets#1131

#46380 introduced a regression in GenAPI preventing the correct loading of the header file when specified. The modified code was assuming the passed string was the contents of the header, but in reality it was the header file path.

The fix consists in reading the contents of the file path in GenAPIApp and then pass that string to CSharpFileBuilder instead.

I also noticed we didn't have any tests to verify different headers, so I added a couple:

• One that passes null, which causes CSharpFileBuilder to fallback to the default string.
• Another one that passes a custom header (the one we normally use for our licensing).
• The rest of the existing tests will keep using empty string (which is considered a valid header).

cc @MichaelSimons

[carlossanlop]

/ba-g failures are unrelated