Support gcdump and diagsession

In addition to minidump, we will support the gcdump and diagsession format.  The benefit is for C# leak investigation which has a much smaller foot print than a full minidump.

Minidump = /diagnostics/processes/{id}/dump?dumptype=(0|1|2)&format=(zip|raw)
Note: zip format will also package matching sos and mscordacwks dls

GCdump = /diagnostics/processes/{id}/dump?format=(zip|raw|diagsession)&maxDumpCountK=(0)
Note: this only lights up if vsdiagagent binaries are installed.

I also make zip related stream more efficient.