Add Dashboard entity and Dashboards collection

[dannyota]

Description of PR

Adds a Dashboard entity and Dashboards collection to splunklib.client, wrapping the data/ui/views/ REST endpoint.

The SDK already provides entity classes for most Splunk resources (indexes, saved searches, inputs, apps, etc.), but dashboards are missing — users currently have to use raw service.get()/service.post() calls. This PR follows the same Entity/Collection pattern used by SavedSearch, Index, Application, and others.

Changes:

• Dashboard(Entity) — with an export() method that returns the XML definition via eai:data
• Dashboards(Collection) — with a create(name, xml) method that passes XML as eai:data
• Service.dashboards property to access the collection
• PATH_DASHBOARDS constant (data/ui/views/)

Usage:

for dash in service.dashboards:
    print(dash.name, dash.export()[:80])

service.dashboards.create("my_dashboard", "<dashboard><label>Test</label></dashboard>")

Testing the changes

Unit tests added in tests/unit/test_dashboard.py (7 tests):

• Entity/Collection subclass checks
• export() returns eai:data content (and empty string when missing)
• create() passes XML as eai:data to the underlying Collection
• create() raises TypeError when XML is omitted

Tested with Python 3.13, Splunk Enterprise 10.4.0. Passes ruff check, ruff format, and basedpyright.

Documentation

Docstrings follow the existing SDK conventions (:return:, :rtype:, :param:). The Service.dashboards property is documented consistently with Service.saved_searches, Service.indexes, etc.

Dependencies and other resources

No external dependencies. Follows the same Entity/Collection pattern as the existing SavedSearch, Index, Application, and Macro classes in client.py.