Enable observability scopes on project access token

What does this MR do and why?

Adds read_observability and write_observability scopes to Project access token. Previously these were only available for Group access tokens. Just like for Group access tokens, these scopes are conditionally available only if observability_tracing is enabled for the root namespace.

All features gated by observability_tracing are currently Experimental.

Closes gitlab-org/opstrace/opstrace#2243 (closed)

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

1. In rails console enable the experiment fully

   Feature.enable(:observability_tracing, <root_group_id>)

2. Visit any project within the <root_group_id> from step 1 and navigate to the access token page, e.g.: http://127.0.0.1:3000/groups/flightjs/Flight/-/settings/access_tokens
3. Click Add new token
4. Verify that read_observability and write_observability are available in the Select scopes section.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.