Add SchemaCompareEndpoint for DacPackage object as parameter

**Is your feature request related to a problem? Please describe.**
I develop a PowerShell module, that provides commands, based on DacFx. It's https://github.com/abbgrade/PsDac.
I'd like to provide a command for schema comparison. Additionally I want only a single method each to load dacpacs, databases and projects and a single method to connect and authenticate to servers.

**Describe the solution you'd like**
A additional implementation of SchemaCompareEndpoint that accepts DacPackage objects.

**Describe alternatives you've considered**
Currently a schema compare requires a connect to the server, an import of a DacPackage, an export to DacPac file and then a compare based on the DacPac. That works but is inefficient and requires a temporary storage.

Start by locating the SchemaCompareEndpoint implementation and the DacPackage type, then review how schema comparison currently accepts server connections and exported DACPAC files. Define the API behavior for accepting a DacPackage directly and verify that comparison no longer requires exporting to temporary storage.

Written by the indexing model from the issue text.