subscriptions: make source type optional for planetary_variable_source function

[charcey]

In draft mode until the next major release is cut!

Related Issue(s):

Closes #

Proposed Changes:

For inclusion in changelog (if applicable):

1. planetary_variable_source constructor doesn't require var_type at all. If using planetary_variable_source please refer to the new function signature here.

Not intended for changelog:

1. Added a test

Diff of User Interface
Previously changes were made to make source type optional. However, in the planetary_variable_source definition, var_type is still required though allowed to be null. This MR changes that such that var_type need not be included at all. This does reorder input arguments, so users should reference the release notes.

Old behavior:

pv_source = planetary_variable_source(
    var_type=None,
    var_id="SWC-AMSR2-X_V5.0_1000",
    start_time=datetime.fromisoformat("2022-12-07T00:00:00Z"),
    end_time=datetime.fromisoformat("2022-12-16T00:00:00Z"),
    geometry={
        "coordinates": [
            [
                [139.56481933, 35.42374884],
                [140.10314941, 35.42374884],
                [140.10314941, 35.77102915],
                [139.56481933, 35.77102915],
                [139.56481933, 35.42374884],
            ]
        ],
        "type": "Polygon",
    },
)

New behavior:

pv_source = planetary_variable_source(
    start_time=datetime.fromisoformat("2022-12-07T00:00:00Z"),
    end_time=datetime.fromisoformat("2022-12-16T00:00:00Z"),
    geometry={
        "coordinates": [
            [
                [139.56481933, 35.42374884],
                [140.10314941, 35.42374884],
                [140.10314941, 35.77102915],
                [139.56481933, 35.77102915],
                [139.56481933, 35.42374884],
            ]
        ],
        "type": "Polygon",
    },
)

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: