Add filter configurations at a per-Source granularity

[ndhaller]

Adds treeless="true" and blobless="true" as attributes to a Combination Source. During edkrepo clone and checkout, if enableSubmodule="true" on the Combination Source, the filter setting applies to the submodules as well.

--treeless and treeless="true" have priority over --blobless and blobless="true" if multiple are used.

During edkrepo clone, --treeless and --blobless do not apply to the submodule filter settings.

Related issues:

https://github.com/tianocore/edk2-edkrepo/issues/239
add filter configurations at a per-Source or per-Remote granularity

https://github.com/tianocore/edk2-edkrepo/issues/247
clone with both --treeless and --blobless encounter git error

[ndhaller]

The changes for adding partial clones to submodules looks good as do the changes to define the partial clone strategy at a per level in the manifest file. Can we separate them into two separate commits?

Added a separate commit for the submodule change.