PS-827: Enable harvester to use dynamic prefix for keys

[wimspaargaren]

Which problem is this PR solving?

This PR addresses the following two issues:

• Enable users to dynamically specify the consul location where a certain key is stored, since Go tags can't be dynamically adjusted
• Reduce redundancy in consul tags

Short description of the changes

A new method is added to the harvester builder, called WithConsulFolderPrefixMonitor. This method allows you to specify an optional prefix of where harvester needs to look for your key in Consul.

This enables users to dynamically adjust the location of where a key is located in Consul. For example, if you deploy a service in multiple clusters, you might want to define the key location based on the deployment using an environment variable. In addition, this removes the redundancy for prefixes of keys.

Old way:

	Key1 sync.String `seed:"dummy value" consul:"services/my-service/key1"`
        Key2 sync.String `seed:"dummy value" consul:"services/my-service/key2"`
        Key3 sync.String `seed:"dummy value" consul:"services/my-service/key3"`

      .......

     WithConsulMonitor("consul-addr", "data-center", "token", time.Second * 42).

New way:

	Key1 sync.String `seed:"dummy value" consul:"key1"`
        Key2 sync.String `seed:"dummy value" consul:"key2"`
        Key3 sync.String `seed:"dummy value" consul:"key3"`

      .......

     WithConsulFolderPrefixMonitor("consul-addr", "data-center", "token", "services/my-service", time.Second * 42).

[mantzas]

LGTM

[c0nstantx]

@wimspaargaren It would be nice if you could update the examples and README with the new Consul setup.

cc @mantzas @Lazaros-Tsigkakos