When building on Ubuntu 22.04 with GCC 11.4.0 and Boost 1.89.0.beta1,
the build failed with:
```
error: ‘floor’ was not declared in this scope
41 | num_char = (int)floor((float)num_char / (float)radix);
| ^~~~~
```
Signed-off-by: Michael Cho <michael@michaelcho.dev>
- Add abstract_http_client.h which http_client.h extends.
- Replace simple_http_client with abstract_http_client in wallet2,
message_store, message_transporter, and node_rpc_proxy.
- Import and export wallet data in wallet2.
- Use #if defined __EMSCRIPTEN__ directives to skip incompatible code.
