plugin: add DirectPost command

[LYC386]

Description

Adds "DirectPost" command for meson plugin.

Using DirectPost command, the payload inside a CurrencyRequest won't be passed to an IChain instance. Instead, It will be directly sent to the RPC Url matching the Ticker. The response by RPC will also be directly sent back to caller without passing to an IChain instance.

The reasons to add DirectPost command are as follows:

• Easier for existing wallet to integrate with Meson. Since most wallets should already handle data sent with RPC by themselves, they only need to wrap the data to a CurrencyRequest.
• Easier to add support to new chains and RPC methods. We only need to update chain factory and currency setting files to support new chains, leaving the json data parts to wallets.
• Existing commands remain unchanged.

DirectPost command can be tested with meson_wallet_demo PR.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• Test A: Test with local mixnet using docker. Use meson_wallet_demo PR to query gas and send a transaction with DirectPost command.
• Test B

Checklist:

• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have checked that any new log messages doesn't inavertedly link compromising information to an external observer about the Meson Mixnet.