Inconsistent Signing Workflow

[shizhMSFT]

There is a conflict in the signing workflow.

• signing-and-verification-workflow.md verifies the certificate chain before signing (steps 1.i and 1.iii).
• specs/plugin-extensibility.md verifies the ceritifate chain after signing (steps 5.i.c and 5.i.d.c).

[rgnote]

I think signing-and-verification-workflow.md was written with local signing in mind and specs/plugin-extensibility.md was written for remote signing. Since we don't have access to the chain before we generate the digital signature in the case of remote signing, we need to verify the chain after signing.