Task: Make secret handling safer with secrecy

[dunxen]

Description

We probably would want the secrecy crate or similar in future to make sensitive values Secret<String>s to reduce the chance of them being logged and only .expose() them where necessary. It also securely wipes it from memory when it's dropped.

Unfortunately, although grpc request headers values can be marked as sensitive (e.g. the macroon header), this doesn't actually do anything nifty like redaction in logging or secure wiping. We probably still want to mark them and then try to manually do something about them.

Steps to Complete

1. Introduce secrecy crate and wrap sensitive values in Secret<T>, .expose()ing them where necessary.

Notes