Move AgentClient into sentry-php

[linear-code]

Instead of having the AgentClient in the agent repository, I think it would be better to have it in sentry-php. This makes sense for the following reasons:

• Applications can use the agent without having it as a dependency, this can be especially useful if multiple applications run on the same machine so only one agent is required
• Fallback is easier to implement as it can just use the regular client with all the SDK informations being available

E2E testing moves to the SDK, the Agent tests can focus on internals

[linear-code]

AGENT-10