Allow override of hovered navbar colors

[ncrocfer]

This PR is a follow up of the #35505 and #35553 ones to customize the navbar color.

When we hover the links with dark colors we can have this behavior with the background color:

navbar_color = #760000
navbar_text_color = #d7cece

With these new settings the user can override the hovered background and text colors:

navbar_hover_color = #4c0102
navbar_text_hover_color = #d7cfce

cc @vchiapaikeo @eladkal @kaxil @jedcunningham

---

^ Add meaningful description above
Read the Pull Request Guidelines for more information.
In case of fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
In case of a new dependency, check compliance with the ASF 3rd Party License Policy.
In case of backwards incompatible changes please leave a note in a newsfragment file, named {pr_number}.significant.rst or {issue_number}.significant.rst, in newsfragments.

[vchiapaikeo]

Nice work! This looks good to me!

[bbovenzi]

Nice!

If you want we could fix the Airflow logo text color by changing the fill here

[ncrocfer]

Indeed this is a good idea to allow the users to customize it too. I check to append a new commit to this PR if you want.

[ncrocfer]

The new navbar_logo_text_color setting (please tell me if you prefer another slug) now allows the customization of the logo text color:

navbar_logo_text_color = #d7cece

The default values in the airflow/config_templates/config.yml file keep the current colors:

[eladkal]

Nice!