* BTCPay Extensions Part 2
This PR cleans up the extension system a bit in that:
* It renames the test extension to a more uniform name
* Allows yo uto have system extensions, which are extensions but bundled by default with the release (and cannot be removed)
* Adds a tool to help you generate an extension package from a csproj
* Refactors the UI extension points to a view component
* Moves some more interfaces to the Abstractions csproj
* Rename to plugins
* new feature: Wallet Receive Page
closes#965
* Conserve addresses by waiting till address is spent before generating each run
* fix tests
* Filter by cryptocode before matching outpoint
* fix build
* fix edge case issue
* use address in keypathinfo directly
* rebase fixes
* rebase fixes
* remove duplicate code
* fix messy condition
* fixes
* fix e2e
* fix
