VSIX: intermediate certificates not included in signature

**Describe the bug**
When signing a VSIX, intermediate certificates --- from both the primary and timestamp certificate chains --- are not included in the signature. This can fail verification in offline environments if machines do not already have those intermediate certificates available at verification time.

[Only the end certificate is included in the signature.](https://github.com/dotnet/sign/blob/dc01dca32471b368ad640358778e172d1bd249f9/src/Sign.Core/Tools/VsixSignTool/XmlSignatureBuilder.cs#L181C1-L183C7)

**Repro steps**
Sign a VSIX and inspect at the XML digital signature.

**Expected behavior**
Every non-root certificate in both the primary and timestamp certificate chains will be included in the signature.

**Actual behavior**
Only end certificates are included.

**Additional context**
VsixSignTool.exe neither includes these certificates nor uses them (if present) in certificate chain validation.