imageserver: add directory dashboard and ability to list images in a directory

[narendraReddy45]

Description & Motivation

This PR implements the below:

• Adds directory dashboard and the ability to list images in a directory in the listDirectories api
• listImages api now accepts query parameter directoryName and returns images matching that directoryName prefix

Approach

The image database currently has a hashmap of image names and their respective metadata. To get all images matching a certain prefix, we could do a linear search on each element and return a list of images.

But this approach may not be scalable, as listImages would loop over len(images) for every call, sort the results, and return.

Optimized the prefix search by creating an index on image names. The index is implemented using a golang slice, maintaining a sorted order of elements and doing a binary search every time a search is needed.

As our dataset grows, we can eventually shift to a trie-based index for better performance. As trie improves insertion and deletion to O(L) where L is the length of the imagename.

Given our dataset N ( e.g.636: from http://imageserver.dev.purestorage.com:6971/metrics/image-count ) is smaller , the performance is comparable and slices are contiguous

Fixes #248

Type of Change

• New feature (non-breaking change which adds functionality)
• Documentation update

How Has This Been Tested?

• deployed local imageserver, created directories and added images and verified the dashboard

Testing Approach

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works