What this PR does

• Use vsixsigntool.exe to sign the VSIX
• Only sign master branch builds
• Update the certificate /sha1
• Include certificate.pfx with the source
• Include vsixsigntool.exe in tools folder (from Microsoft.VsSdk.VsixSignTool, v16.2.29116.78)

Notes

I used the following command to find /sha1 9c5a6d389e1454f2ed9ee9419cdf743689709f9c for vsixsigntool.exe:

> certutil -p PASSWORD -dump .\certificate.pfx
================ Certificate 0 ================
================ Begin Nesting Level 1 ================
Element 0:
Serial Number: 0409181b5fd5bb66755343b56f955008
Issuer: CN=DigiCert Assured ID Root CA, OU=www.digicert.com, O=DigiCert Inc, C=US
 NotBefore: 22/10/2013 12:00 PM
 NotAfter: 22/10/2028 12:00 PM
Subject: CN=DigiCert SHA2 Assured ID Code Signing CA, OU=www.digicert.com, O=DigiCert Inc, C=US
Non-root Certificate
Cert Hash(sha1): 92c1588e85af2201ce7915e8538b492f605b80c6
----------------  End Nesting Level 1  ----------------
No key provider information
Cannot find the certificate and private key for decryption.

================ Certificate 1 ================
================ Begin Nesting Level 1 ================
Element 1:
Serial Number: 09144b52c8330f257d2512daaaa1e545
Issuer: CN=DigiCert SHA2 Assured ID Code Signing CA, OU=www.digicert.com, O=DigiCert Inc, C=US
 NotBefore: 05/02/2020 12:00 AM
 NotAfter: 09/02/2023 12:00 PM
Subject: CN=GitHub, Inc., O=GitHub, Inc., L=San Francisco, S=California, C=US
Non-root Certificate
Cert Hash(sha1): 9c5a6d389e1454f2ed9ee9419cdf743689709f9c
----------------  End Nesting Level 1  ----------------
  Provider = Microsoft Enhanced Cryptographic Provider v1.0
Encryption test passed
CertUtil: -dump command completed successfully.