feat: Encrypting the content of the rln credential file

[staheri14]

Problem

The rln credential file contains privacy-sensitive membership information of the user and should be encrypted. Currently, it is a plaintext file. This would not cause any serious issue as long as we are running on the Ethereum testnet, but when moving to the mainnet then membership credentials will have actual financial value.
Relevant part of the code https://github.com/status-im/nwaku/blob/982fb08c7735c131ecacc801f21ac56d15286567/waku/v2/protocol/waku_rln_relay/waku_rln_relay_utils.nim#L1131

Suggested solution

The rln credentials should be encrypted before being written to the rlnCredentials.txt file. A potential solution is to use Keystore similar to how it is used by nimbus.

Alternatives considered

NA

Additional context

Acceptance criteria

• At a high level, the rlnCredentials.txt should only contain encrypted credentials but not plain ones. The decryption power should be given to the user to unlock the file when needed. The use of Keystore is one way to achieve so.

[rymnc]

Addressed with the usage of waku keystore in #1466