ClickOnce: propose new signing algorithm

[dtivel]

After working on #773 for some time, it's clear that Sign CLI's ClickOnce signing algorithm needs improvement.

• ClickOnce: add ClickOnce signing algorithm spec #843

Merge into feature branch https://github.com/dotnet/sign/tree/dtivel/clickonce in order:

• PR 1: https://github.com/dotnet/sign/tree/dtivel/clickonce-interfaces
• PR 2: https://github.com/dotnet/sign/tree/dtivel/clickonce-app-impl-tests
• PR 3: https://github.com/dotnet/sign/tree/dtivel/clickonce-signer-core
• PR 4: https://github.com/dotnet/sign/tree/dtivel/clickonce-cli-integration