Add support for statically allocated buffer in etdump (#1295)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1295

This diff adds support for generating etdump in a statically allocated buffer that is passed in by the user. In many embedded use cases users want fine grained control on where etdump is placed and want to avoid any dynamic memory allocations.

All that the user's need to do is pass in a Span into the `ETDumpGen` constructor. This Span needs to contain the pointer to the statically allocated buffer and its size.

Reviewed By: vmpuri

Differential Revision: D50854912

fbshipit-source-id: c41f3347124b31152990aa0f4459ad1e3b169532