Metachain exposing control RPCs through feature gate

As per the discussion from #97 and #106, we've decided to prepackage defid with metachain for production and expose control RPCs only for testing. We can set a flag at build-time for both metachain and ain, so that metachain exposes the RPC and ain enforces a -meta-url when -meta flag is missing.

Note that it won't even be possible to set this URL in a production build.

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[fuxingloh]

/triage accepted

Note that it won't even be possible to set this URL in a production build.

I don't think we should restrict the feature based on the release channels. We should make it network-based. The current convention on defid is to allow customizable flags when used in conjunction with -regtest network.

https://github.com/DeFiCh/ain/blob/94e6b978f58b55809f2dc4929a4b32280b99bd7b/src/init.cpp#L483-L509

Might need further clarification here. If we pass RPC URL, then defid doesn't need to have prepackaged metachain, in which case we can get rid of linking to libmetachain dependency entirely, so it's better to have it as a build time flag right? Or do you mean that defid executable will always have metachain prepackaged and if we pass RPC URL, it will switch to using RPC client instead of calling FFI?

[fuxingloh]

Might need further clarification here. If we pass RPC URL, then defid doesn't need to have prepackaged metachain, in which case we can get rid of linking to libmetachain dependency entirely, so it's better to have it as a build time flag right? Or do you mean that defid executable will always have metachain prepackaged and if we pass RPC URL, it will switch to using RPC client instead of calling FFI?

The latter.

The defid executable will always have metachain prepackaged and if we pass RPC URL flag with the -regtest flag, it will switch to using RPC client instead of calling FFI.