Support for multiple contexts in kubeconfig

[alexeykazakov]

Fixes #83

1. New contexts_list tool for listing all available contexts in the cubeconfig.
2. All tools now have an optional contex param. If not specified then the default context is used.

Here is an example of context_list result:

Available Kubernetes contexts (3 total, current: cluster-1):

Format: [*] CONTEXT_NAME -> CLUSTER_SERVER_URL
        (* indicates the current active context)

Contexts:
─────────
* cluter-1 -> https://api.cluster-1.com:443
  cluter-2 -> https://api.cluster-2.com:443
  cluter-3 -> https://api.cluster-2.com:443
─────────

Usage:
To use a specific context with any tool, add the 'context' parameter:
Example: {"name": "pods_list", "arguments": {"context": "cluster-1"}}`

[iamsudip]

This is a useful tool, waiting for it to get merged