feat(CENG-689) Add generic upstream support

[BartoszBlizniak]

Add Generic upstream support to the provider.

Example usage:

resource "cloudsmith_repository_upstream" "gradle_distributions" {
    name            = "Gradle Distributions"
    namespace       = "${data.cloudsmith_organization.my_organization.slug_perm}"
    repository      = "${resource.cloudsmith_repository.my_repository.slug_perm}"
    upstream_type   = "generic"
    upstream_url    = "https://services.gradle.org"
    upstream_prefix = "distributions"
}

[Copilot]

Pull request overview

This PR adds support for Generic upstream repositories to the Terraform provider, allowing users to configure generic upstream sources like Gradle distributions. The changes include updating the cloudsmith-api-go dependency from v0.0.50 to v0.0.54, implementing Generic upstream CRUD operations, adding comprehensive test coverage, updating documentation, and fixing a bug in the team resource where the description field now correctly uses nullableString instead of optionalString.

Changes:

• Added Generic upstream support with all CRUD operations (Create, Read, Update, Delete)
• Updated cloudsmith-api-go dependency to v0.0.54 to support Generic upstream API
• Fixed team resource description field to use nullableString for proper API compatibility

Reviewed changes

Copilot reviewed 5 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
go.mod	Updated cloudsmith-api-go dependency from v0.0.50 to v0.0.54
go.sum	Added checksums for cloudsmith-api-go v0.0.54
docs/resources/repository_upstream.md	Added Generic upstream documentation with example and updated parameter table
cloudsmith/resource_team.go	Fixed description field to use nullableString instead of optionalString for proper nullable handling
cloudsmith/resource_repository_upstream.go	Added Generic constant, UpstreamPrefix constant, Generic upstream CRUD operations, and schema definition for upstream_prefix field
cloudsmith/resource_repository_upstream_test.go	Added comprehensive test coverage for Generic upstream with basic, update, and import scenarios

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