This adds a context.Context parameter to NewFs and related calls.
This is necessary as part of reading config from the context -
backends need to be able to read the global config.
Before this change crypt would not calculate hashes for files it was
uploading. This is because, in the general case, they have to be
downloaded, encrypted and hashed which is too resource intensive.
However this causes backends which need the hash first before
uploading (eg s3/b2 when uploading chunked files) not to have a hash
of the file. This causes cryptcheck to complain about missing hashes
on large files uploaded via s3/b2.
This change calculates hashes for the upload if the upload is coming
from a local filesystem. It does this by encrypting and hashing the
local file re-using the code used by cryptcheck. For a local disk this
is not a lot more intensive than calculating the hash.
See: https://forum.rclone.org/t/strange-output-for-cryptcheck/15437Fixes: #2809
