Add previous custom types contract back in as simple_custom_types

[sisuresh]

What

#313 rewrote the custom-types contract. This PR adds that contract back in under a slightly different name so this documentation stays current https://developers.stellar.org/docs/build/smart-contracts/example-contracts/custom-types. If this PR is accepted, the docs will need to link to this contract instead.

It would be ideal if the current custom-types contract could be renamed or moved instead, but I'll defer to @BlaineHeffron / @fnando / @chadoh on that because the CLI uses it.

[leighmcculloch]

It would be ideal if the current custom-types contract could be renamed or moved instead, but I'll defer to @BlaineHeffron / @fnando / @chadoh on that because the CLI uses it.

+1 the custom types example being resurrected in this pr should be the custom types example because it is simpler and easier to understand by a much wider variety of folks with different backgrounds. @sisuresh Can you rename the example in this PR to custom_types to replace the example?

I'm not really sure the example that was added in #313 really belongs in examples, even under a different name, because it isn't hyper-focused on a simple example. It's actually a test vector for the js-stellar-sdk, and the examples repo is not intended to host test vectors. The other repos like the rs-soroban-sdk, and rs-soroban-env, that need specific test vectors host their own test vectors internally. Test vectors are often poor examples due to other noise and conflicting intent.

For the moment I suggest we rename the #313 custom type example to custom_type_more_types or something else while we give the js-stellar-sdk space to figure out a path forward.

Potentially we need a soroban-test-vectors repo where non-example test-vectors can live. cc @Shaptic