feat: implement extension in testnet

[chungquantin]

Before preparing the release for adding pop api to testnet:

• refactor: api function index #124
• Final decision if anything needs to be changed here
• docs: readme pop api #294
• Important for people checking out the pop api
• Make a decision if we have to make a release for the pop api crate (or any other). Contract developers will use it and one pro is having them adding a crate in stead of a git dependency. Con is that it requires time. Think about more pros and cons and have the team make a decision.
• Ensure feat: psp22 trait contract example with drink tests #300 will not cause additional changes to the runtime and thus an extra release.
• Integration tests pass for testnet runtime: chore(api): testnet integration tests #368

[al3mart]

An update:

• A decision was made to continue towards bringing the extension and api to testnet with refactor: api function index #124 as it is at this point.

• pop-api README: docs: pop-api v0 fungibles READMEs #365

• pop-api crate won't be released yet. We first need to see if FungiblesError is finally included.

• feat: psp22 trait contract example with drink tests #300 Won't introduce new changes into the runtime.

[Daanvdplas]

Could we please update the description with the newly identified requirements. This is:

• Implement extension in testnet runtime
• Integration tests working also with testnet runtime

Please others I missed.