feat: support custom favicons

[kkedziak-splunk]

Issue number: ADDON-81027

PR Type

What kind of change does this PR introduce?

• Feature
• Bug Fix
• Refactoring (no functional or API changes)
• Documentation Update
• Maintenance (dependency updates, CI, etc.)

Summary

Changes

TA will now show custom favicon if it is set. If it is not, then the default is shown.

User experience

Users will see custom favicon if it is set.

Checklist

If an item doesn't apply to your changes, leave it unchecked.

Review

• self-review - I have performed a self-review of this change according to the development guidelines
• Changes are documented. The documentation is understandable, examples work (more info)
• PR title and description follows the contributing principles
• meeting - I have scheduled a meeting or recorded a demo to explain these changes (if there is a video, put a link below and in the ticket)

Tests

See the testing doc.

• Unit - tests have been added/modified to cover the changes
• Smoke - tests have been added/modified to cover the changes
• UI - tests have been added/modified to cover the changes
• coverage - I have checked the code coverage of my changes (see more)

Demo/meeting:

Reviewers are encouraged to request meetings or demos if any part of the change is unclear

[kkedziak-splunk]

From left: Search App, Example TA with this change, Example TA without this change

[hetangmodi-crest]

Should we also update docs to let users know where they should place the favicon in this format or the dimensions, or simply redirect them to Splunk dev docs?

[hetangmodi-crest]

Is it possible to update SmartX and test out this change?

[soleksy-splunk]

Is it possible to update SmartX and test out this change?

Are there any new tests in SmartX that would enable testing this one?

The idea is to introduce testing functionality in SmartX and then introduce it also here, so we actually have the ability to test which icon is rendered etc.