Improve Soroban upgrade script

[sisuresh]

• Specify which settings to upgrade with a better UX than updating the hardcoded setting in the script. Maybe a JSON file with the upgrades will work well.
• Update the network used (futurenet vs standalone vs pubnet) on the command line.
• Fund the source account with friendbot if it doesn't exist.
• Add an endpoint in stellar-core that takes a proposed ConfigUpgradeSetKey and returns what the upgrade diff will be.
• Remove dependency on python SDK and build transactions in stellar-core. (Added after the issues we had on 9/15)

[sisuresh]

Related to #3822

[leighmcculloch]

This issue is a blocker for the following issue that is impacting contract developers today:

• Remove use of soroban high limit override and replace with actual config upgrade quickstart#508

[leighmcculloch]

• build transactions in stellar-core

It would be convenient to have this in stellar-core, but also since the lack of this script is causing problems today, and the next release of stellar-core is not for sometime, it would be ideal if we build this tool behavior into its own tool that we could ship today/soon.

[leighmcculloch]

This issue is a blocker for the following feature issue:

• Configure archival behavior quickstart#512

[sisuresh]

This issue is about improving the Python upgrade script (which we're going to get rid of). I'm closing it issue in favor of #3822.