lncli: add command to create new macaroon

[guggero]

As discussed in #1147 there is a need to create macaroons with custom permissions set since the three existing macaroon files admin.macaroon, invoice.macaroon and readonly.macaroon aren't fine-grained enough.

A new gRPC method named NewMacaroon is added:

• Introduces a new permission entity named macaroon (could be used for macaroon based RPCs mentioned in Add support for accounting-based macaroons #291 too).
• write access to the entity macaroon is necessary to call the method NewMacaroon.
• The admin.macaroon gets write access to the entity macaroon.
• A new command is added to lncli named newmacaroon that calls this gRPC method.
• As parameters to the NewMacaroon method a list of entity/action pairs for the allowed operations can be passed.

Example:

lncli newmacaroon --permission=invoices/write --permission=invoices/read --save_to=~/.lnd/custom-invoice.macaroon --timeout=10

Creates a macaroon that is valid for reading and writing invoices during the next 10 seconds.

Closes #283, #1147, #3516.

NOTE for release notes: Users will need to delete or move their admin.macaroon, readonly.macaroon and invoices.macaroon before starting 0.9, otherwise they won't get regenerated macaroons that have the required permission (macaroon:generate) to mint custom macaroons.

[vegardengen]

This one actually conflicts with #1147

Would it make sense to make this into one common PR? Or merge one, then rebase/fix conflicts after that?

[guggero]

If either of the two PRs is merged, I'll rebase the other one. The functionality should not conflict, both commands have their usefulness IMO, even if there is overlap in some of the command arguments.

[wpaulino]

LGTM 💥

I think another nice follow-up would be to allow lncli to use a custom macaroon through a flag as hex to prevent having to save it to a file.

[guggero]

@Roasbeef asked me to rename the service to BakeMacaroon.