fix(jsonrpc): use padded-base64 for payload encoding

[LNSD]

This PR includes code and API cleanup and adds tests covering the base64 module. A typical gardening PR 🪴

Note
The v0.15.0 is working as expected: Payload must be encoded using regular base64 (with padding).

---

Regarding the RFC discussion about the JSON-RPC message payload encoding using base64 (here):

Ok... There was some confusion on my side: I thought the payload was encoded in base64 URL-friendly format without padding. This is not what was happening. It is encoded in regular base64 with padding. It seems I implemented it the right way from the very beginning.

So the v0.15.0 is working as expected: Payload must be encoded using regular base64 (with padding).

I have updated this PR so it is clear that the encoding must be with padding.

[fryorcraken]

I can confirmed this fixes the issues previously reported in #1564
With this PR, nwaku know accepts NodeJS Buffer's base64 string encoding.

[jm-clius]

LGTM thanks!