[FSSDK-8838] feat(odp-cache): Adds support for odp cache.

[yasirfolio3]

Summary

• Provides 2 out of the box ODP cache including in-memory and redis.
• in-memory will use the builtin LRU cache with user provided size and timeout.
• Adds support for custom odp cache.
• If no odp cache is provided, agent will use go-sdk's in built LRU Cache with default size and timeout.

Out of Box ODP Cache Usage

1. To use the in-memory ODPCache, update the config.yaml as shown below:

   ## configure optional ODP Cache
   odp:
   segmentsCache:
   default: "in-memory"
   services:
     in-memory: 
       ## 0 means cache will be disabled
       size: 0
       ## timeout after which the cached item will become invalid.
       ## 0 means records will never be deleted
       timeout: 0s

2. To use the redis ODPCache, update the config.yaml as shown below:

   ## configure optional ODP Cache
   odp:
   segmentsCache:
   default: "redis"
   services:
     redis: 
       host: "your_host"
       password: "your_password"
       database: 0 ## your database
       timeout: 0s

Custom ODPCache Implementation

To implement a custom odp cache, followings steps need to be taken:

1. Create a struct that implements the cache.Cache interface in plugins/odpcache/services.
2. Add a init method inside your ODPCache file as shown below:

   func init() {
   myCacheCreator := func() cache.Cache {
       return &yourCacheStruct{
       }
   }
   odpcache.Add("my_cache_name", myCacheCreator)
   }

3. Update the config.yaml file with your ODPCache config as shown below:

## configure optional ODPCache
odp
  segmentsCache:
    default: "my_cache_name"
    services:
      my_cache_name: 
        ## Add those parameters here that need to be mapped to the ODPCache
        ## For example, if the ODP struct has a json mappable property called `host`
        ## it can updated with value `abc.com` as shown
        host: “abc.com”

• If a user has created multiple ODPCache services and wants to override the default ODPCache for a specific sdkKey, they can do so by providing the ODPCache name in the request Header X-Optimizely-ODP-Cache-Name.

• Whenever a request is made with a unique sdkKey, The agent node handling that request creates and caches a new ODPCache. To keep the ODPCache type consistent among all nodes in a cluster, it is recommended to send the request Header X-Optimizely-ODP-Cache-Name in every request made.

Tests

• Unit tests added.
• FSC should pass.

Ticket

FSSDK-8838