feat: add filter hook jwt_auth_iss to customize iss

[dayan3847]

Description

Please include a summary of the change or changes and which issue is fixed. Please also include relevant motivation and context.

Fixes #(issue)

Type of change

• 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 not to work as expected)
• This change requires a documentation update

How has this been tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration and if you use specific plugins.

• Describe test A
• Describe test B

Test Configuration:

• WordPress version
• PHP version
• Plugin name and version
• Plugin B name and version

Checklist:

• My code follows the style guidelines of this project (WordPress code standards)
• I have performed a self-review of my own code
• I have commented on my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (if needed)
• My changes generate no new warnings
• I have described how I made my tests that prove my fix is effective or that my feature works

[dayan3847]

Issue: "jwt_auth_bad_iss response - Headless WordPress #301"