AdHoc filters: Allow typing the same custom value

[ashharrison90]

• passes a custom isValidNewOption function to the valueSelect
  • allows any non-zero-length string as a valid option
  • this prevents showing a No options found message when entering the same custom value as was initially present
• adds a unit test to prevent regressions

Fixes https://github.com/grafana/hyperion-planning/issues/25

📦 Published PR as canary version: 4.17.2--canary.723.8999827741.0

:sparkles: Test out this PR locally via: ```bash npm install @grafana/scenes@4.17.2--canary.723.8999827741.0 # or yarn add @grafana/scenes@4.17.2--canary.723.8999827741.0 ```

[grafanabot]

:rocket: PR was released in v4.17.3 :rocket: