Soroban: Support for Constructors

[tareknaser]

This PR adds support for user-defined constructors in Soroban, specifically for constructors with no arguments. Note that constructors with arguments are not yet supported (see comments for details). Part of #1672

Other changes include:

• Refactored code in tests/soroban.rs to allow providing constructors when registering contracts on the chain.
• Added tests for constructors in tests/soroban_testcases/constructor.rs.