feat(ion-header): add basic UI for ionic theme (#29629)

Issue number: internal

---------

<!-- Please do not submit updates to dependencies unless it fixes an
issue. -->

<!-- Please try to limit your pull request to one type (bugfix, feature,
etc). Submit multiple pull requests if needed. -->

## What is the new behavior?
<!-- Please describe the behavior or changes that are being added by
this PR. -->

- Added new common header partial, to be used by ionic theme and ios/md
- Renamed header.scss to header.native.scss. This inherits the common
created above and its used only on ios and md partial
- Created new partial for ionic theme.
- Added ionic theme to basic e2e tests.

## Does this introduce a breaking change?

- [ ] Yes
- [x] No

<!--
  If this introduces a breaking change:
1. Describe the impact and migration path for existing applications
below.
  2. Update the BREAKING.md file with the breaking change.
3. Add "BREAKING CHANGE: [...]" to the commit description when merging.
See
https://github.com/ionic-team/ionic-framework/blob/main/docs/CONTRIBUTING.md#footer
for more information.
-->

## Other information

<!-- Any other information that is important to this PR such as
screenshots of how the component looks before and after the change. -->

[Sample Header
Screen](https://ionic-framework-rcfex2fep-ionic1.vercel.app/src/components/header/test/basic?ionic:theme=ionic)

---------

Co-authored-by: Maria Hutt <thetaPC@users.noreply.github.com>
Co-authored-by: ionitron <hi@ionicframework.com>
Co-authored-by: Maria Hutt <maria@ionic.io>