feat: implement `access_token` auth method

[0x416e746f6e]

Description

Please include a summary of the changes and the related issue. Please also include relevant motivation and context. List any dependencies that are required for this change.

Fixes #660

Type of change

Please delete options that are not relevant.

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Improvement (improve existing resource(s) or datasource(s))
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• [ ] Test A: make testacc TESTARGS="-run TestAccDataSourceXxxxYyyyZzzzz_basic"
• [ ] Test B: make testacc TESTARGS="-run TestAccDataSourceXxxxYyyyZzzzz_basic"

Test Configuration:

• Terraform version: terraform version: Terraform v1.8.5
• Existing HCL configuration you used:

  
  #
  # this is what I really did.  but setting up the whole vault, and oauth2 plugin, 
  # and configuring ovh auth engine, is not really necessary.  just passing the 
  # token received from `/auth/oauth2/token` as hardcoded `access_token` argument
  # will also do
  #
  data "vault_generic_secret" "this" {
  path = "secret/oauth2/self/ovh-eu"
  }

provider "ovh" { endpoint = var.ovh_endpoint

access_token = data.vault_generic_secret.this.data["access_token"] }

data "ovh_me" "this" {}

output "ovh_me" { value = data.ovh_me.this }

# Checklist:

- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my code
- [x] I have commented my code, particularly in hard-to-understand areas
- [x] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings or issues
- [ ] I have added acceptance tests that prove my fix is effective or that my feature works
- [ ] New and existing acceptance tests pass locally with my changes
- [x] I ran succesfully `go mod vendor` if I added or modify `go.mod` file