[clang-format] Added new option IndentExternBlock

Reviewers: MyDeveloperDay, krasimir, klimek, mitchell-stellar, Abpostelnicu

Patch By: MarcusJohnson91

Reviewed By: MyDeveloperDay, Abpostelnicu

Subscribers: sylvestre.ledru, Abpostelnicu, cfe-commits

Tags: #clang, #clang-format, #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D75791