make jsonrpc api namespaces extensible

[yihuang]

Description

Closes: #1052, #1037

An example of usage: https://github.com/crypto-org-chain/cronos/pull/432/files#diff-7acb7332ad576a49e7685db72282959006f156c06a708828d8b0b187e311ffe1R32

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[fedekunze]

I like the approach but ideally you shouldn't be able to override existing namespaces, just add new ones. Can you update the implementation to support that?

[yihuang]

I like the approach but ideally you shouldn't be able to override existing namespaces, just add new ones. Can you update the implementation to support that?

good idea, added a method to register extra namespace.

[fedekunze]

ACK, minor comments