[Copilot] meshPlatform resource implementation

[j0g3sc]

The meshcloud docs mention support for the 'meshPlatform' object.
Implement the meshPlatform resource in this terraform provider!

To do so follow these instructions:

• Add a go file within the client directory for the client code.
• Add a meshPlatform resource to the internal/provider package.

For the structure of meshPlatform with all possible configurations read
https://docs.dev.meshcloud.io/api/index.html#mesh_platform and https://docs.dev.meshcloud.io/api/index.html#mesh_platform_configs

Use a similar structure for the schema as the other resources.

Do not use any new additional go libraries.

Use CU-86c55h4xp for all commits.