fix Property 'availability' is missing in type 'CustomHeightmapTerrai…

[Tim-S]

…nProvider' but required in type 'TerrainProvider' when using with typescript

Description

When try using CustomHeightmapTerrainProvider from a small typescript toy-project I got a

Property 'availability' is missing in type 'CustomHeightmapTerrainProvider' but required in type 'TerrainProvider'

added simple default Implementation in CustomHeightmapTerrainProvider (basically same as in EllipsoidTerrainProvider)

to make the basic CustomHeightmapTerrainProvider from the doc work

Issue number and link

Testing plan

Author checklist

• I have submitted a Contributor License Agreement
• I have added my name to CONTRIBUTORS.md
• I have updated CHANGES.md with a short summary of my change
• I have added or updated unit tests to ensure consistent code coverage
• I have update the inline documentation, and included code examples where relevant
• I have performed a self-review of my code

[github-actions]

Thank you for the pull request, @Tim-S!

✅ We can confirm we have a CLA on file for you.

[ggetz]

Thanks @Tim-S for the fix! Overall this looks good, just one change requested from me.

Please also add a summary of your change to CHANGES.md, as this does fix functionality for those using TS.

[ggetz]

Awesome, thanks @Tim-S!