feat: add client_id() header method and multi-platform search support

[stark256-spec]

Closes #49, closes #80.

Changes

client_id() method on Query (fixes #49)

Adds a client_id(client_id=None) convenience method to the Query base class, analogous to token() and bearer_token(), that sets the Client-Id header described in the CMR Search API docs.

• Without an argument (or None): sets Client-Id: python_cmr-<version> so that NASA Operations can identify requests from this library automatically.
• With a string argument: sets Client-Id: <your-app>/python_cmr-<version>, appending the library version for traceability.

query = CollectionQuery().client_id()             # Client-Id: python_cmr-0.14.0
query = CollectionQuery().client_id("my-tool")   # Client-Id: my-tool/python_cmr-0.14.0

Multi-platform support in platform() (fixes #80)

Updates GranuleCollectionBaseQuery.platform() to accept either a single string or a list of strings, matching CMR's native support for filtering by multiple platforms.

# single platform (existing behaviour preserved)
query.platform("Terra")

# multiple platforms
query.platform(["Terra", "Aqua"])

Tests

• 3 new tests in tests/test_queries.py covering client_id() default, custom name, and header isolation.
• 2 new tests (one per GranuleQuery and CollectionQuery) for multi-platform lists.
• Existing single-platform assertions updated to reflect the new list storage format.