[8.x] Make ResponseSequence macroable

[SjorsO]

This PR makes the ResponseSequence class macroable. This class is used by Http::fakeSequence().

My first use-case is that some APIs always need to retrieve an access token before any calls can be made. Currently I always push a fixture of the access token response every time I call Http::fakeSequence(). Custom macros can clean this up:

Http::fakeSequence()
    ->pushFile($this->fixtures.'dhl/access-token-01.json')
    ->pushFile($this->fixtures.'dhl/tracking-01.json');

// After this PR

Http::fakeSequence()
    ->pushDhlAccessTokenResponse()
    ->pushFile($this->fixtures.'dhl/tracking-01.json');

Macros can also be used to craft custom responses. This is a use-case from another project of mine:

$checkoutUrl = 'http://example.com/'.Str::random();

Http::fakeSequence()
    ->pushMollieCheckoutResponse($checkoutUrl);

$this->postCheckout()
    ->assertRedirect($checkoutUrl);