Add SkySatVideo as a valid data api item type

[jreiberkyle]

Related Issue(s):

Closes #956

Proposed Changes:

For inclusion in changelog (if applicable):

1. Separate data api item type validation from orders api item type validation, add SkySatVideo as a valid data api item type

Not intended for changelog:

Diff of User Interface

Old behavior:

planet-client-python$> planet data search --help
Usage: planet data search [OPTIONS] ITEM_TYPES

...

  Valid entries for ITEM_TYPES: MYD09GA|MOD09GA|Sentinel1|Landsat8
  L1G|PSScene|SkySatCollect|MYD09GQ|PSOrthoTile|MOD09GQ|REScene|Sentinel2L1C|S
  kySatScene|REOrthoTile

planet data search skysatvideo
Usage: planet data search [OPTIONS] ITEM_TYPES
Try 'planet data search --help' for help.

Error: Invalid value for 'ITEM_TYPES': item_type - 'skysatvideo' is not one of 'REOrthoTile', 'REScene', 'MOD09GA', 'Landsat8L1G', 'SkySatScene', 'MYD09GQ', 'MYD09GA', 'MOD09GQ', 'PSScene', 'SkySatCollect', 'Sentinel1', 'PSOrthoTile', 'Sentinel2L1C'.

New behavior:

SkySatVideo listed as a valid entry for ITEM_TYPE

planet-client-python$> planet data search --help
Usage: planet data search [OPTIONS] ITEM_TYPES

...

  Valid entries for ITEM_TYPES: MYD09GA|MOD09GA|Sentinel1|SkySatVideo|Landsat8
  L1G|PSScene|SkySatCollect|MYD09GQ|PSOrthoTile|MOD09GQ|REScene|Sentinel2L1C|S
  kySatScene|REOrthoTile

planet data search skysatvideo
<VALID_SKYSATVIDEO_ITEM_DESCRIPTIONS>

PR Checklist:

• [] This PR is as small and focused as possible
• [] If this PR includes proposed changes for inclusion in the changelog, the title of this PR summarizes those changes and is ready for inclusion in the Changelog.
• [] I have updated docstrings for function changes and docs in the 'docs' folder for user interface / behavior changes
• [] This PR does not break any examples or I have updated them

(Optional) @mentions for Notifications:

[sgillies]

@jreiberkyle I made a suggestion to use less code. 👍 on joining the sets like you've done.

[sgillies]

@jreiberkyle since get_data_item_types() below is essentially a one line function, how about this instead? Let's avoid creating new functions when they're not strictly required at the moment.

Suggested change

	return _validate_field(item_type, get_data_item_types(), 'item_type')
	return _validate_field(item_type, get_item_types() | {'SkySatVideo'}, 'item_type')

[jreiberkyle]

ohhh wait, I was going to say yes absolutely but the data CLI uses get_data_item_types() to list the valid item types in the help. So removing get_data_item_types() would require the same one-liner to be added to planet/cli/data.py as well. Introducing get_date_item_types() was chosen to keep the "hacktastic" code in one place within specs.

[jreiberkyle]

but looking at the code, there are a lot of opportunities to get rid of short-term variables and extra lines of code. I did what I could while keeping get_data_item_types()