Allow to pass custom activation function in `policy_kwargs`

Added the possibility to pass a custom activation function through the policy_kwargs argument when creating the following models: TD3, SAC, DDPG and DQN (like in sablebaseline3).

Description

Taking inspiration from stablebaseline, I've put the common code of the critic under the sbx\common\policy.py file since the critic code both in the sac module and in td3 module was the same.

Minor changes to .gitignore and Makefile have been made.

Motivation and Context

[x] I have raised an issue to propose this change (#37 )

closes #37

Types of changes

[ ] Bug fix (non-breaking change which fixes an issue)
[x] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to change)
[ ] Documentation (update in the documentation)

Checklist:

[x] I've read the CONTRIBUTION guide (required)
[ ] I have updated the changelog accordingly (required).
[ ] My change requires a change to the documentation.
[x] I have updated the tests accordingly (required for a bug fix or a new feature).
[ ] I have updated the documentation accordingly.
[x] I have reformatted the code using make format (required)
[x] I have checked the codestyle using make check-codestyle and make lint (required)
[x] I have ensured make pytest and make type both pass. (required)
[ ] I have checked that the documentation builds using make doc (required)

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araffin / sbx