Release Agent API as a composer package

This is regarding the requirements to be able to release a version for the apm-agent-php.

### Requirements
- Easy to release
- CI should be orchestrator
- Sensitive credentials should be stored in Vault
- Release to https://packagist.org/
- One script to be able to release independently of the CI, so if required it should be possible to release locally. [composer](https://getcomposer.org/) with packagist.org
- Tag event-based, when a particular tag release is created then the automation should do the same stages as done with PRs/Branches in addition to the release.

### Actions
- [ ] Release automation with composer
- [x] Create user in packagist and store credentials in vault
- [ ] Write down the release process in its RELEASING.md file
- [ ] Automate the process in the pipeine

### Prerequisites
- The master build pipeline should be stable.

### Questions
- Does packagist support snapshot releases? In other words, if we would like to potentially publish the binary for every PR, is that something we can achieve with that service? We already have in place artifactory, so I could ask if they support the PHP central repo. This is required for the ongoing iniative of moving to CD.