feat: add paytm credentials and setup in admin/settings

[uds5501]

Fixes #3320

Changes proposed in this pull request:

• Add PayTM credentials in settings model.
• Add validations for new fields in the project.
• Add fields following other payment gateways

Checklist

• I have read the Contribution & Best practices Guide.
• My branch is up-to-date with the Upstream development branch.
• The acceptance, integration, unit tests and linter pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

[mrsaicharan1]

@uds5501 So just for clarification, we're trying to enable scenarios where paytm can be disabled and keys are retained. Hope that helps! I've made the changes on the server too. So you can check it out.

[uds5501]

@kushthedude @mrsaicharan1 took care of these in the current iteration, please check

[kushthedude]

Adding DO-NOT-MERGE until the server one is merged

[uds5501]

The server PR : fossasia/open-event-server#6249 has been merged.
@mrsaicharan1 please review and merge