Add support for async signature computation

[rpuch]

In WebCrypto API, all the operations, including signature computation, are async and return promises. Trying to implement RSA-SHA1 signature method, I found out that it was impossible to do because the library always uses getSignature() in a synchronous manner.

The idea is to add an async method that is able to work with async getSignature() (and getBodyHash()) methods.

Please note that I did not add anything to the TS file. Could you please give me a hint: what changes are to be added there?

[ddo]

pls check the ci test fail

[rpuch]

@ddo done, the tests have passed

[voxxit]

Bump?