Conan install API endpoints

Implement the API endpoint (details below) needed in order to successfully run conan install and view package info from the GitLab Conan registry.

This is the 4th of 6 steps to implementing the Conan API https://gitlab.com/gitlab-org/gitlab-ee/issues/8248:

1. #12567 (closed) - Skeleton API - Ping endpoint
2. #12568 (closed) - Conan authentication endpoints
3. #13345 (closed) - conan upload endpoints
4. #13350 (closed) conan install endpoints
5. #13347 (closed)conan search endpoints
6. conan info endpoints (all endpoints for conan info are implemented in previous steps)
7. #31941 (closed) conan remove endpoints

Endpoints to implement in this issue:

• GET /v1/conans/{name}/{version}/{username}/{channel}/download_urls v1_recipe_download_urls
• GET /v1/conans/{name}/{version}/{username}/{channel}/packages/{package_id}/download_urls v1_package_download_urls
• GET /v1/files/{path} v1_updown_file

conan install makes a series of API calls to the registry (aka remote). Here is an example of the series of calls made:

GET http://server:9300/v1/ping # implemented in #12567
GET http://server:9300/v1/conans/Hello/0.1/user/channel/download_urls
GET http://server:9300/v1/files/Hello/0.1/user/channel/0/export/conanmanifest.txt?signature=*******
GET http://server:9300/v1/files/Hello/0.1/user/channel/0/export/conanfile.py?signature=**********
GET http://server:9300/v1/conans/Hello/0.1/user/channel/packages/0ab9fcf606068d4347207cc29edd400ceccbc944/download_urls
GET http://server:9300/v1/files/Hello/0.1/user/channel/0/package/0ab9fcf606068d4347207cc29edd400ceccbc944/0/conaninfo.txt?signature=**********
GET http://server:9300/v1/conans/Hello/0.1/user/channel/packages/0ab9fcf606068d4347207cc29edd400ceccbc944 # implemented in #13345
GET http://server:9300/v1/conans/Hello/0.1/user/channel/packages/0ab9fcf606068d4347207cc29edd400ceccbc944/download_urls
GET http://server:9300/v1/files/Hello/0.1/user/channel/0/package/0ab9fcf606068d4347207cc29edd400ceccbc944/0/conanmanifest.txt?signature=**********
GET http://server:9300/v1/files/Hello/0.1/user/channel/0/package/0ab9fcf606068d4347207cc29edd400ceccbc944/0/conaninfo.txt?signature=**********
GET http://server:9300/v1/files/Hello/0.1/user/channel/0/package/0ab9fcf606068d4347207cc29edd400ceccbc944/0/conan_package.tgz?signature=**********