In-memory lnd credentials

[orbitalturtle]

At LNDK (https://github.com/lndk-org/lndk) some users need the option to pass in macaroon/tls credentials to LNDK directly. Since this fedimint fork of tonic_lnd seems more actively maintained recently, we've been using it instead.

Looks like @Evanfeenstra already implemented something like this: Kixunil#30 Just curious if this PR would be welcome on this fork.