Ensure keychain/ASM/1P secret values are obfuscated with a comment

[alecthomas]

Something that has occurred multiple times now is that users manually modify and/or create secrets in a format that can't be read by FTL (ie. not JSON). This is not surprising, as the values are relatively human readable, eg. a password might be "Deau34dWKWLCMWE8ymCN", however if the human edited value isn't JSON encoded correctly, FTL will fail to decode it.

I think we should add a comment to values, and obfuscate them so it's not trivial to edit them. eg. the above might be:

# This secret is managed by "ftl secret set", DO NOT MODIFY
IkRlYXUzNGRXS1dMQ01XRTh5bUNOIg==

Ideally this would not be per-provider, but generically applied. This might be fairly straightforward, if the resolver does this encoding, and the providers just store/retrieve the raw bytes.

[gak]

Similar: #1772