Launch droplet in a specific project

[antondalgren]

This pull request adds support for specifying a DigitalOcean Project ID when creating droplets with the DigitalOcean builder. This allows users to launch droplets directly into a specific DigitalOcean project, improving resource organization and management. The changes include updates to the configuration structs, documentation, and droplet creation logic.

DigitalOcean Project ID support:

• Added a new optional project_id field to the DigitalOcean builder configuration (Config and FlatConfig structs in config.go and config.hcl2spec.go). [1] [2]
• Updated the HCL2 spec and mapstructure to recognize project_id as a valid configuration key.
• Modified the droplet creation logic to include project_id when launching a new droplet and to store it in the build state. (step_create_droplet.go) [1] [2]

Documentation updates:

• Added project_id to the documented list of available configuration keys in both the main README and partial documentation files. (README.md, Config-not-required.mdx) [1] [2]

Depends on: digitalocean/godo#881
Closes #174