Restore --tipset functionality to forest-cli snapshot export

[jdjaustin]

Issue summary

The --tipset flag was removed as part of PR #2638 but we still need this functionality to compare Forest snapshot exports byte-for-byte with Lotus snapshot exports; the approach has been to download a Lotus snapshot, then export the Forest snapshot at the exact same height using the --tipset flag with forest-cli snapshot export.

Other information and links

Here is the exact location where the --tipset flag previously appared in the codebase: https://github.com/ChainSafe/forest/pull/2638/files#diff-a302ffc3bbc97bbfc1a2ce05d64d36ee32b6c9dbf285a318cec375e71735f947L40