Change v2 X-Planet-App header to reflect SDK

[sarasafavi]

Related: #336

X-Planet-App is used internally by Planet; the list of allowed values is limited. Work with the team to determine new value for SDK requests (and, potentially, for CLI vs SDK requests)

[cholmes]

+1 to CLI vs SDK release. @sarasafavi to figure out what we need from D&D team to track cli vs sdk.