Allow item types specified to be case insensitive in Data API and CLI

[kevinlacaille]

Related Issue(s):

Closes #599

Proposed Changes:

For inclusion in changelog (if applicable):

1. Added validation for item_type in the Data client, which allows item types to be case insensitive. Functions affected:

• client: search(); CLI: search
• client: create_search(); CLI: search-create
• client: update_search(); CLI: search-update
• client: get_stats(); CLI: stats

Not intended for changelog:

Diff of User Interface

Old behavior:

> planet data search psscene                                     
Error: {"general": [], "field": {"item_types": [{"message": "Not all item types requested exist or are accessible"}]}}

New behavior:

> planet data search psscene                                     
{"_links": {"_self": "https://api.planet.com/data/v1/item-types/PSScene/items/20230328_161317_49_241a", "assets": "https://api.planet.com/data/v1/item-types/PSScene/items/20230328_161317_49_241a/assets/", ...}

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:

[cholmes]

Looks great, thanks!

[jreiberkyle]

works great! Could you update the PR desc to include the CLI commands that are affected by this as well?