chore: Update IDM schema

[tilenkavcic]

This updates IDM schema with updated custom object shards mentioned here.
It also brings the schema up to date with the latest features we introduced.

https://app.devrev.ai/devrev/works/ISS-189380

[Copilot]

Pull Request Overview

This PR enhances the initial mappings schema by adding support for custom object shard creation modes, introducing fallback handling, updating terminology capitalization, and embedding version metadata.

• Added a discriminator and shard-mode enum for custom object mappings.
• Defined a new fallback object and integrated it into typed references.
• Introduced devrev_metadata_version and format_version fields and standardized “DevRev” casing.

Comments suppressed due to low confidence (3)

initial_mappings_schema.yaml:52

• The shard-mode enum only includes create_shard, but the description mentions assigning an existing shard as well. Consider adding an enum value for existing shards or updating the description to match the supported modes.

        enum:

initial_mappings_schema.yaml:316

• The new fallback property in airdrop-recipe-filter-typed-reference lacks a description. Please add a brief description explaining its purpose and usage.

            fallback:

initial_mappings_schema.yaml:521

• The format: text declaration for format_version may not conform to OpenAPI format standards. Consider removing the format field or replacing it with a valid format or pattern definition.

        format: text