Allow increased rate limit access to OpenAPI specs

[jamietanna]

In https://github.com/ecosyste-ms/ecosystems-go we're looking to set up Renovate with Vendir to manage the OpenAPI spec updates, similar to how I've done it in https://github.com/rootlyhq/rootly-go

While attempting to do so, Vendir's vendir sync command is failing due to:

Fetching: specs + packages.yaml (http from https://packages.ecosyste.ms/docs/api/v1/openapi.yaml)

vendir: Error: Syncing directory 'specs':
  Syncing directory 'packages.yaml' with HTTP contents:
    Downloading URL:
      Expected 200 OK, but was '429 Too Many Requests'

AFAICT, Vendir doesn't set a custom user-agent, nor set custom headers

I believe that decreasing rate limit requirements on the OpenAPI specs should be more reasonable

This change also needs to be done for the other specs

• https://packages.ecosyste.ms/docs/api/v1/openapi.yaml
• https://repos.ecosyste.ms/docs/api/v1/openapi.yaml
• https://commits.ecosyste.ms/docs/api/v1/openapi.yaml
• https://issues.ecosyste.ms/docs/api/v1/openapi.yaml