[lib] Call the getPublicKey callback.
[tools] Add contextMap config value.

Allow configs to map a context URL to an object or context file. This is
useful when using experimental contexts that are not yet hosted at their
final URL.
[lib] Improve signature timestamp checking.

- Add option to not check timestamp time.
- Add option to select max timestamp delta.
[tools] Properly pass request options.
[tools] Add URL suport for sign and verify.