README page for snowflake plugin / tiny refactor of snowflake plugin

[babourine]

No description provided.

[Copilot]

Pull Request Overview

This PR introduces comprehensive documentation for the Snowflake plugin and performs a small refactor of its internal context handling.

• Adds a dedicated README for the snowflake plugin with usage, configuration, and best practices
• Refactors the Snowflake command code by extracting snowflakeClusterContext and snowflakeJobContext, removing the unused DSN field
• Updates the main README.md to link to the new Snowflake plugin documentation

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
plugins/snowflake/README.md	New plugin documentation covering setup, job submission, result retrieval, and security
internal/pkg/object/command/snowflake/snowflake.go	Split contexts into separate structs, removed unused fields, updated unmarshalling
README.md	Added hyperlink for the Snowflake plugin entry in the plugin overview table

Comments suppressed due to low confidence (3)

internal/pkg/object/command/snowflake/snowflake.go:27

• Typo in error message: 'invalida' should be 'invalid'.

ErrInvalidKeyType         = fmt.Errorf(`invalida key type`)

plugins/snowflake/README.md:67

• The example uses 'type:snowflake-prod', which doesn't match the defined cluster tags ('type:snowflake', 'data:prod'). Update the criteria to align with the actual tags or clarify the matching logic.

"cluster_criteria": ["type:snowflake-prod"],

internal/pkg/object/command/snowflake/snowflake.go:30

• [nitpick] This struct is empty and may not be needed; consider removing it or adding a comment to explain its intended use.

type snowflakeCommandContext struct{}