Update OnPolicyAlgorithmJax & PPO to support custom rollout_buffer_class

[Trenza1ore]

Description

Fixes #89

Just a small add to support rollout_buffer_class and rollout_buffer_kwargs arguments in constructor of PPO and OnPolicyAlgorithmJax, like in Stable Baselines3.

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[araffin]

is dict obs space supported for PPO?
if not, maybe put an assert (and add a small test to check it fails)

[Trenza1ore]

MultiInputPolicy indeed haven't been implemented for PPO yet. I added the assertion and new test case for it. Although @araffin maybe it's a bit better to raise NotImplementedError here and in OffPolicyAlgorithmJax._setup_model()?

[araffin]

thanks for the PR, overall LGTM =) just a small comment

[araffin]

LGTM, thanks =)