Add --url option to the `truffle exec` command

[sukanyaparashar]

PR description

This PR adds --url option to the truffle exec command that connects to a specified url. See #5701.

Testing instructions

To test this option, you can clone this Test Project with a simple storage contract and a test script.

1. Run a ganache instance in a terminal.
2. Run truffle compile inside a truffle project in a separate terminal.
3. Run truffle exec test-script.js --url http://127.0.0.1:8545. It should execute the script and deploys a new instance of the contract and returns the value of the state variable that is set with a constructor.

Documentation

• [x] I thought about documentation and added the doc-change-required label to this PR if documentation updates are required.