[sarif] Minimal generation of a sarif object + introducing mdr_register.ml

src/plugins/markdown-report/Makefile

src/plugins/markdown-report/Report_markdown.mli

src/plugins/markdown-report/markdown.ml

src/plugins/markdown-report/markdown.mli

src/plugins/markdown-report/md_gen.ml

src/plugins/markdown-report/md_gen.mli

src/plugins/markdown-report/mdr_register.ml

src/plugins/markdown-report/mdr_register.mli

src/plugins/markdown-report/sarif.ml

src/plugins/markdown-report/sarif_gen.ml