Add fetchPermission utility documentation

[spencerstock]

What changed? Why?

Added new fetchPermission utility function documentation to the Base Account SDK spend permissions documentation. This function allows retrieving a single spend permission by its hash identifier, complementing the existing fetchPermissions function that retrieves multiple permissions.

Key changes:

• Created new documentation page for fetchPermission utility
• Updated fetchPermissions documentation to clarify that the spender parameter is optional
• Updated coinbase_fetchPermissions RPC method documentation to reflect optional spender parameter
• Added fetchPermission to the navigation structure in docs.json
• Added references to the new utility in the spend permissions guide

Notes to reviewers

The new fetchPermission utility provides a more direct way to retrieve individual permissions when you have a permission hash, which is useful for tracking specific permissions across sessions.

How has it been tested?

Documentation changes only - verified markdown formatting and code examples are syntactically correct.

[cb-heimdall]

✅ Heimdall Review Status

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