Add support for partial checkout filters (#1396)

* added filter option & tests

* added build file

* fix test oversight

* added exit 1

* updated docs to specify override

* undo unneeded readme change

* set to undefined rather than empty string

* run git config in correct di

---------

Co-authored-by: Cory Miller <13227161+cory-miller@users.noreply.github.com>

src/git-source-settings.ts

@@ -29,6 +29,11 @@ export interface IGitSourceSettings {
    */
   clean: boolean
 
+  /**
+   * The filter determining which objects to include
+   */
+  filter: string | undefined
+
   /**
    * The array of folders to make the sparse checkout
    */