This adds a `GetCustomCaveatCondition` function that returns the custom
caveat condition for a given macaroon and caveat name. Previously there
was no function for getting the custom caveat condition from a macaroon,
only for setting one.
The way the macaroon bakery library lnd uses works is that one has to
register a Checker method for each caveat name that should be supported.
Since we want to allow fully customizable custom caveats we add another
layer of naming to the caveat by splitting the condition of the "outer"
caveat into two pieces, the custom caveat name and the actual custom
caveat condition.
The custom Checker function only checks that the format is correct and
that there is a handler available for a custom condition. It does not
check the condition itself, however. If the passed in acceptor signals
acceptance of a custom caveat then the bakery accepts the macaroon as a
whole (given its signature, standard caveats and permissions are all
correct) and assumes that another component down the line will make sure
the actual custom condition of a caveat is valid.
This commit fixes a go 1.15 vet check.
In doing so it uncovers that the time caveat check is actually reversed.
Since we should check that the caveat is added, we should only fail the
check when the caveat prefix is not equal.
This commit is a step to split the lnwallet package. It puts the Input
interface and implementations in a separate package along with all their
dependencies from lnwallet.
