[Maps] TMS allows invalid URL to be added as layer

[kindsun]

Currently there are no checks or validation that a TMS source is valid prior to adding. A user can input a completely erroneous URL and then advance to layer settings with no warning or indication the URL is invalid. This also means that if a URL were to become invalid (like if a server goes down) there would be no indication to the user. Related to #30588

We should be preventing a user from adding invalid layers and also showing an indicator in the TOC when a TMS layer isn't available.

To Test:

1. Go to Add layer > Tile map service
2. Add the following mistyped URL: http://c.tile.staen.com/watercolor/{z}/{x}/{y}.jpg (it's missing the m in stamen)
3. Skip attribution and click Add layer

[elasticmachine]

Pinging @elastic/kibana-gis (Team:Geo)

[thomasneirynck]

Closing. It's hard to validate if a server works for TMS. They may respond with errors for a given raster-tile, but that would still be valid behavior.