Add `stellar tx send-via-core` command

[leighmcculloch]

What problem does your feature solve?

The existing stellar tx send command submits transactions exclusively through Soroban RPC's sendTransaction endpoint. There is no way to submit a transaction directly to stellar-core, or horizon.

Submitting directly to stellar-core is useful when:

• Ops work setting up new networks where an RPC isn't running but a classic tx needs sending.
• Debugging transaction submission issues where you want to isolate whether the problem is in RPC or core.
• Operating in environments where Soroban RPC is unavailable but stellar-core is accessible.
• Testing core directly during development or network upgrades.

What would you like to see?

A new stellar tx send-via-core subcommand that submits a transaction envelope directly to a stellar-core instance, bypassing Soroban RPC.

The command would follow the same pattern as stellar tx send:

stellar tx send-via-core [TX_XDR] --core-url <url>

• Accept a base-64 encoded transaction envelope XDR from a positional argument, file, or stdin (same as stellar tx send).
• Accept a --core-url flag (or resolve it from network configuration) pointing to a stellar-core HTTP endpoint.
• Submit the transaction to stellar-core's tx HTTP endpoint.
• Print the response from core.

What alternatives are there?

• Use curl to manually POST the transaction XDR to a stellar-core instance's /tx endpoint.
• Submit via stellar tx send through RPC, which internally forwards to core anyway.