Enhance endpoint security for /api/v1/users/sign_in with randomization

[7riumph]

Part of epic #3942

What type of user does this affect?

• volunteers

How should it operate? ⚙️🛠️

Endpoint can now output randomized token and refresh_token on successful /api/v1/sign_in request.

Additionally, since tokens now expire, user model has token_expires_at and refresh_token_expires_at datetimes.

Acceptance Criteria

• Expands the user model to handle both the token and refresh token, as well as their expiration times.
• Creates functions to generate randomized tokens in user.rb and set default expiration to 7 hours and 30 days
• Updates session controller and base controller with new details if applicable

Edit: Made api_credentials table and put randomized functions in new models/api_credential.rb that belongs to user

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