feat: Added optional start process at elementId

[daghsentinel]

closes #581

Background

We have maintained a separate fork of this library that has the ability to start a process at a specific elementId. To avoid keeping this fork up to date manually, we'd like to share the functionality as it seems more people are missing it and at the same time bring the functionality into the main line of the client.

[CLAassistant]

All committers have signed the CLA.

[daghsentinel]

Any chance of getting a review on this one, @ChrisKujawa? I'd appreciate your comments.

[daghsentinel]

Bump

[ChrisKujawa]

Let's use a similar method name and docs as in the Java API
https://github.com/camunda/camunda/blob/5b65051a91066850d72b3bfb7fb54fe23abcd6d2/clients/java/src/main/java/io/camunda/client/api/command/CreateProcessInstanceCommandStep1.java#L117C1-L126C82

/**
   * Overrides the default start position of the process. Calling this method will make the
   * process start at the given {@code elementId}, if possible. This method can be called more
   * than once to simultaneously start at different elements in different branches of the process.
   *
   * @param elementId the id of the BPMN element where to start the process instance
   * @return the builder for this command. Call {@link #send()} to complete the command and send *
   *     it to the broker.
   */
  CreateProcessInstanceCommandStep3 startBeforeElement(final String elementId);