feat: add start instructions to gateway mapping

[pihme]

Description

Adds mapping for start instructions to gateway.

(Applies to both CreateProcessInstance as well as CreateProcessInstanceWithResult - the second case is calling the mapping code of the first case)

Related issues

closes #410

Definition of Done

Code changes:

• The changes are backwards compatibility with previous versions
• If it fixes a bug then PRs are created to backport the fix

Testing:

• There are unit/integration tests that verify all acceptance criterias of the issue
• New tests are written to ensure backwards compatibility with further versions
• The behavior is tested manually

Documentation:

• Javadoc has been written
• The documentation is updated

[github-actions]

Unit Test Results

  47 files    47 suites   1m 40s ⏱️
107 tests 107 ✔️ 0 💤 0 ❌
340 runs  340 ✔️ 0 💤 0 ❌

Results for commit c928c1b.

[remcowesterhoud]

👍 LGTM