Add customer timespan filter to dashboard

[neumann-nico]

Why
I really love the new dashboard! It simplifies steps that I was previously doing manually for the analysis of my flows.
To unleash the full potential, I need a filter that allows me to customize the timespan (e.g. set to one week).

What

• Add a button "custom" next to the 8h/24h/1w button
• Opens a little menu with a dropdown (hours/weeks/months), a text field and an apply button
• When the user clicks on apply the ?span= parameter in the URL is adjusted to the set value
• User will see the dashboard with the custom timespan

Notes
I'm talking about this filter

Slack chat: https://prefect-community.slack.com/archives/C0192RWGJQH/p1689580372318809