Sending large payload crashes EventPipe

[sywhang]

Currently, EventPipe's buffer manager is capable of allocating up to 1MB buffer to save event payloads. But the EventPipeFile's block which is used to flush the buffer has a maximum size of 100 KB. This causes an event whose payload is small enough to fit in a EventPipe buffer ( < 1MB) but is larger than the max size of the block (> 100 KB) to crash with a call stack like this:

`00 (Inline Function) --------`-------- KERNELBASE!DebugBreak [minkernel\kernelbase\debug.c @ 143]
01 00000034`3e87f7a8 00007ffc`31c9c8f7 KERNELBASE!wil::details::DebugBreak+0x2 [internal\sdk\inc\wil\resultmacros.h @ 1888]
02 00000034`3e87f7b0 00007ffc`31c97e6c coreclr!file_write_event_to_block+0xa7 [D:\workspace\_work\1\s\src\native\eventpipe\ep-file.c @ 249]
03 00000034`3e87f800 00007ffc`31c951d9 coreclr!ep_file_write_event+0xfc [D:\workspace\_work\1\s\src\native\eventpipe\ep-file.c @ 460]
04 00000034`3e87f860 00007ffc`31c9b56d coreclr!ep_buffer_manager_write_all_buffers_to_file_v4+0xb9 [D:\workspace\_work\1\s\src\native\eventpipe\ep-buffer-manager.c @ 1180]
05 (Inline Function) --------`-------- coreclr!ep_buffer_manager_write_all_buffers_to_file+0x17 [D:\workspace\_work\1\s\src\native\eventpipe\ep-buffer-manager.c @ 1056]
06 00000034`3e87f8e0 00007ffc`31c9d8a1 coreclr!ep_session_write_all_buffers_to_file+0x4d [D:\workspace\_work\1\s\src\native\eventpipe\ep-session.c @ 435]
07 00000034`3e87f910 00007ffc`31c99f47 coreclr!streaming_thread+0xc1 [D:\workspace\_work\1\s\src\native\eventpipe\ep-session.c @ 56]
08 00000034`3e87f960 00007ffc`62037974 coreclr!ep_rt_thread_coreclr_start_func+0x17 [D:\workspace\_work\1\s\src\coreclr\vm\eventing\eventpipe\ep-rt-coreclr.h @ 1923]
09 00000034`3e87f990 00007ffc`64d3a2d1 kernel32!BaseThreadInitThunk+0x14 [base\win32\client\thread.c @ 64]
0a 00000034`3e87f9c0 00000000`00000000 ntdll!RtlUserThreadStart+0x21 [minkernel\ntdll\rtlstrt.c @ 1163]`

Note that in the previous C++ implementation, this would've led to the event simply getting dropped, but in the current C implementation we are using EP_UNREACHABLE() that puts us at this state. This is a regression from Preview 1 to Preview 2 which made the transition from C++ to C implementation of EventPipe.

cc @josalem

[sywhang]

Fixed via #50600.