Add filtering of skills in the bounties page

### User Story
As a developer, I would like to go to the bounty task corresponding to my skills, so I would like to add filters for skills in [the bounty list page](https://gitcoin.co/explorer), such as front-end (javascript\react\vue), back-end (go\python\solidity), design, product, etc.

### Why Is this Needed
Many developers don't have a full range of skills and there are many development languages.
So adding filter criteria for skills on [the bounty list page](https://gitcoin.co/explorer) will allow developers to quickly match to areas they are good at.

### Description
*Type*: Feature request

### Current Behavior
The current page does not have a filter for corresponding skills.

### Expected Behavior
Add a filter for the skill on the right side of the content area.

### Definition of Done
Developers can quickly filter tasks for all skills on [the bounty list page](https://gitcoin.co/explorer)

### Additional Information
Hope to add better participation experience.
The feature request comes from discussion in one meetup of the Gitcoin Chinese community on July 1st, 2021