[18.0][IMP] resource_booking: let requesters choose a resource combination

[JohnnyVM]

Motivation

Implements the resource_booking roadmap item and the generic proposal in #230. Portal requesters currently receive an organizer-fixed or automatically assigned combination without a way to choose among eligible combinations.

Behavior

• Let a tokenized or signed-in requester choose a combination configured on the booking type, or retain automatic assignment.
• Filter offered slots with existing resource/calendar availability methods; carry the choice through month navigation and confirmation.
• Validate the requested combination, modification deadline, and current slot availability on the server before confirming. Preserve fixed organizer assignments and the existing no-choice behavior.
• Expose the reschedule action for confirmed bookings and document the new portal option.

Verification

Odoo 18 resource_booking tests with OCA web and Website: 46 tests passed, 0 failures and 0 errors, including browser tours. One cold-start browser tour timed out initially; its focused rerun and a full suite rerun both passed. Python and QWeb syntax checks passed. The portal XML now passes the repository Prettier configuration.

The barbershop website, pricing, and Spanish-specific localization remain outside this upstream PR.

Closes #230.

[OCA-git-bot]

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