feat: Add custom headers to requests

[DarronEngelbrecht]

This PR allows adding custom headers to the request through the headers method or PEST_STRESS_TEST_OPTIONS variable. We needed this for authentication.

You can use it like this...

$result = stress('example.com')->headers([
    'Authorization' => 'Bearer '. $this->bearerToken,
])

Added a test using reflection as I didn't want to create a getter just for testing.

@nunomaduro @nahime0 - This addresses issue #9 and can help address issue #20 but should probably be merged with PR #19

[ihorchepurnyi]

Hello, this is a super useful PR! Could you please clarify when you’ll be able to review or merge it?

[ihorchepurnyi]

@nunomaduro Could you please take a look? I’m just trying to understand if there are any blockers before merging

[devajmeireles]

@DarronEngelbrecht Could you please rebase this to the 3.x branch?

[DarronEngelbrecht]

@devajmeireles - Thanks for your work on Pest. Have cherry picked these commits to the 3.x branch and opened a second PR hope that works for you, https://github.com/pestphp/pest-plugin-stressless/pull/24/files

[devajmeireles]

Closed in favor of #24