[Feature] Added MongoDB caching support

[gravi2]

This PR does following:

1. Adds support for using MongoDB for caching.
2. Includes some minor DRY changes in the rendertron.ts for cache config selection.
3. README and configuration document changes.

Below is sample configuration that should be used for MongoDB cache.

{
    "timeout": 60000,
    "cache": "mongodb",
    "cacheConfig": {
        "cacheDurationMinutes": 21600,
        "cacheMaxEntries": -1,
        "mongoURI": "mongodb://192.168.86.22:27017/render-cache"
    }
}

[gravi2]

@AVGP Please consider this PR for adding support for MongoDB cache.