fix file and cmd secret source inputs

[kpenfound]

Reported here: #6829 (comment)

Based on my testing, the following did not work:

dagger call foo --token env:TOKEN
echo $TOKEN > token.txt
dagger call foo --token file:./token.txt

Breaking out the file parsing code, the token.txt would be parsed with a newline character at the end.

This adds TrimSpace() to file and cmd secret inputs. TrimSpace() trims leading and trailing unicode whitespace characters. I can't think of any cases where this would have unintended consequences for secrets.

[sipsma]

Hm yeah I agree this is likely what users would want 99.99% of the time and fairly annoying to require them to deal with themselves in userspace (i.e. get secret plaintext, strip newlines, create new modified secret).

I feel like there must be some obscure corner case where this isn't desired, but I can't think of any concretely either. And push comes to shove if we need to support the 0.01% of cases we can always add some kludge like
--token=file-raw:/some/token/file/where/trailing/newlines/matter
or similar.

cc @vito @shykes @jedevc in case you know of any concrete cases where this wouldn't be desired

[vito]

Yeah seems worth the trade-off to me.

I do remember being bitten in the past by something that wasn't happy with a TLS certificate without a trailing linebreak. But I'd rather say whatever tool that was is wrong. Unless that tool is openssl or something. 😅