LSAT aware client interceptor

[guggero]

This PR makes the loop daemon aware of the LSAT authentication scheme.
An unary client gRPC connector is added that intercepts server responses and handles payment challenges sent by the server.
An LSAT token is automatically acquired and paid by the interceptor and then stored in the file .loop/<network>/lsat.token.

[Roasbeef]

Nice work! It's exciting how close we are to deploying LSAT for Loop! One thing that came up, but which shouldn't block this PR is adding a new loopd-level RPC to allow the user to obtain and examine their current LSAT token. This may be useful for user support, and also just some insight into the user's current level, their tier, etc.

[wpaulino]

Should we also consider maintaining a history of all LSATs paid for accounting purposes?

[guggero]

I updated the store to keep old tokens and not just overwrite them.
Also there is now a command that lists all tokens (current and expired) in the command line.

The reason I implemented a file based token store is so people wanting to integrate the interceptor into their code base don't necessarily need a boltDB.
The question is now, should we also add a boltDB compliant store for loopd itself or go with the file based store for now?

[Roasbeef]

LGTM 🐲

Left some final comments w.r.t updated godoc comments as well as naming for the new command.

[wpaulino]

Linter failing with:

lsat/store_test.go:76:11: S1005: should omit value from range; this loop is equivalent to `for key := range ...` (gosimple)
	for key, _ := range tokens {
	         ^
lsat/store_test.go:111:11: S1005: should omit value from range; this loop is equivalent to `for key := range ...` (gosimple)
	for key, _ := range tokens {

[wpaulino]

This is pretty much good to go! Linter is failing again with:

lsat/interceptor.go:125: File is not `goimports`-ed (goimports)
	// we know it's not valid.

[guggero]

Fixed last nits, this should be good to merge. @alexbosworth want to take a final look?

[Roasbeef]

LGTM 🤺

Once we're ready on the server-side, we'll need to update the client to use our new proxy port.