GIN-341: add certificate setting to teams gateway account gateway configuration

[alyssamw]

Description

This new certificate setting specifies the certificate used for Gateway TLS interception. It replaces and deprecates the custom certificate setting.

https://developers.cloudflare.com/api/operations/zero-trust-accounts-get-zero-trust-account-configuration

Has your change been tested?

Added to unit tests in teams_accounts_test.go

Types of changes

What sort of change does your code introduce/modify?

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.
• This change is using publicly documented in cloudflare/api-schemas
  and relies on stable APIs.

[github-actions]

changelog detected ✅

[github-actions]

This functionality has been released in v0.99.0.

For further feature requests or bug reports with this functionality, please create a new GitHub issue following the template. Thank you!