Add go templating to OIDC role templates

[EtienneBruines]

Is your feature request related to a problem? Please describe.
I am wanting to generate a JWT using the Identity secret engine, but am unable to add a prefix/suffix to my strings, nor am I able to modify capitalization or anything.

Describe the solution you'd like
Something like this to become possible as a template:

{
    "id": {{"job:"+identity.entity.aliases.auth_jwt_7818b1f8.metadata.nomad_job_id}}
}

Describe alternatives you've considered

• Attempting to modify the value of the metadata when it goes from the auth token to the metadata (is not supported, only direct JSON pointers to a single value).
• Attempting to modify the value (or add a new value) at the Nomad side of things that includes a more complex string (is not supported, Add arbitrary claims to a job's workload identity nomad#19438)
• Attempting to not require use of this, but not all applications offer this flexibility (e.g. SurrealDB requires the id field to be of the format table:id)

Explain any additional use-cases

• Adding a suffix/prefix
• Modify capitalization (all-caps, all-lowercase)
• Combining multiple metadata fields into one

Additional information
It doesn't have to be go templating - it can be anything that's a bit more flexible than this. Even allowing this would be helpful:

{
    "id": "job:{{identity.entity.aliases.auth_jwt_7818b1f8.metadata.nomad_job_id}}"
}