Add the ability to provide arbitrary custom header claims

[cwirving]

As discussed in #4 this adds the ability to easily provide arbitrary additional header claims when calling signJWT().

The approach is the same as for the JWT body, allowing extra header claims of any type.

Additionally, the tests have been updated to show that extra header claims of can be provided without any linting errors and that they round-trip correctly.

Finally, fix the missing await in one of the tests which was tripping up in CI builds, but not locally.