Add `dash0 export` command to write assets to individual files

[mmanciop]

Provide a dash0 export command that exports each asset to a separate file in a directory provided as input.
Each asset should be stored in a separate file, with the name based on the identifier rather than the display name (assets can be renamed in Dash0, which would not play nicely with GitOps toolkits).

This is complementary to <asset> list -o yaml, which outputs all assets as a single multi-document YAML stream (see #67).